Since the release of ChatGPT in 2022, within just 2 and a half years, it has evolved into an indispensable tool for communication, creativity, and productivity that many people and businesses rely on daily. During this time, OpenAI has been regularly releasing new models, from GPT-3.5 to GPT-4.5 and the lesser-known OpenAI-o1 and OpenAI-o4 mini. With constant updates, it is easy to get overwhelmed if these models are just incremental improvements or revolutionary breakthroughs or if the company is moving towards dividing models by task types like coding, brainstorming, reasoning, visual recognition, and more.
In this article, we will discuss the evolution of ChatGPT. We will also cover the difference between some of the most popular models like GPT-4, GPT-4o, and GPT-4.5, discuss their features, and determine which of them are relevant today and may suit your needs best.
The Origin of ChatGPT
ChatGPT was released on November 30, 2022, sparking the AI revolution. In the first five days, the bot gained 1 million users and, by February 2023, became the fastest-growing consumer app in the history of the Internet. The chatbot was based on the GPT 3.5 model and could remember context, answer questions, and hold natural conversations.
It was not the very first OpenAI model, though. Before its release, OpenAI launched GPT-1 in 2018, GPT-2 in 2019, and GPT-3 in 2020. GPT-1, OpenAI’s first language model, had 117 million parameters and showed that AI could learn language patterns using a transformer model. GPT-2 had 1.5 billion parameters and could write text that sounded human. It was great at tasks like translation and summarization. It wasn’t perfect, but it proved the idea worked. GPT-3, with 175 billion parameters, was a massive leap. It could write essays, answer questions, and even write code. All three models preceding the ChatGPT launch were important milestones in the development of AI technologies and became the foundation for ChatGPT and many AI apps we use today.
The Evolution of ChatGPT
OpenAI's model GPT-3.5 became available in the form of a chatbot, marking the point at which the world became aware of the AI assistant. Early users were absolutely impressed by its capability to provide quick and clear answers to a wide range of questions. The model has found wide application in various fields, from automated chatbots, customer support systems, and text content creation to idea generation and logical problem-solving. In February 2023, just three months after its launch, OpenAI introduced a paid version of the service — ChatGPT Plus. For $20 per month, users got access to advanced models, priority service, and experimental features.
While the world was just starting to be amazed by the recently launched ChatGPT, based on GPT-3.5 and its capabilities, OpenAI released GPT-4 for Plus subscribers in March 2023. This model not only improved the basic characteristics of the chatbot but also introduced a revolutionary multimodality feature. ChatGPT could now work not only with text but also with images. The new model became capable of browsing websites on demand to pull up relevant information. In contrast, previously, the AI was limited to the moment of the last data update, which was initially in September 2021. At the same time, in spring 2023, OpenAI began actively developing mobile platforms, releasing an iOS app with support for synchronizing chats and voice input based on Whisper technology in May 2023. Two months later, in July 2023, they also released an Android app.
In May 2024, OpenAI announced the GPT-4o (with "o" standing for "omni") model, which again took the chatbot's capabilities to a new level. Key improvements included real-time data processing, enhanced support for over 50 languages, and the ability to understand visual and audio information. Like GPT-4, GPT-4o model can integrate information collected from relevant websites into its responses, rather than relying solely on what was learned during training. The model is optimized to work with both structured and unstructured data, and it better perceives ambiguous or contradictory information, making it a universal tool for many industries. It is especially well-suited for everyday tasks, such as writing letters, summaries, brainstorming, and proofreading reports.
This model is now regarded as the latest flagship model of OpenAI, as intelligent as GPT-4 but faster and more versatile. It is important to note that it is available even to free users. While there are restrictions on the number of queries, it is still quite suitable for most everyday tasks. If you do not need to solve super-complex tasks but want to use artificial intelligence to analyze text, write articles, or solve other tasks, GPT-4o will be an excellent choice. The Plus subscription provides access to a larger number of queries, significantly expanding the model's capabilities.
The GPT-4o is also presented in a GPT4o-mini version for those who don't need the power of the full GPT-4o and want to optimize it for different use scenarios. The mini model is perfect for basic tasks like text generation, question answering, or even simple data analysis tasks. It isn't designed for complex calculations, but it's good enough for typical everyday tasks. The GPT-4o mini is available to Plus and Pro subscribers, as well as free users once they've reached their GPT-4o query limit. It's a great option for those who don't want to pay for the Pro subscription but still frequently use AI to solve everyday tasks.
In September 2024, OpenAI released another model - OpenAI-o1. This model is not an incremental improvement of the previous model; rather, it introduces a new approach to information processing. OpenAI-o1 has a unique ability to reason before giving an answer, which is especially important when solving complex scientific problems. Trained based on the "chain of thought" optimization algorithm, it allows users to solve problems step by step and significantly outperforms previous models, such as GPT-4o, in tests. Compared to GPT-4o, it is more focused on complex programming, math, and science problems.
However, OpenAI developed OpenAI-o1 as a complement to GPT-4o, not as a replacement. It takes much longer for OpenAI-o1 to generate an answer, but it produces more thoughtful and reasoned output. The model literally imitates the process of thinking - its reasoning can be followed step by step in the text, much as if an expert was thinking about the solution out loud. This reasoning approach makes it particularly suitable for financial analysis, strategic office planning, or complex theoretical problems. Initially, the model was also available with the OpenAI-o1-mini version, a cost-efficient reasoning model, but this model is currently deprecated. It's worth noting that OpenAI-o1 is only available to paid subscribers, i.e., users with a Plus or Teams subscription are limited to 50 messages per week.
On December 5, 2024, OpenAI introduced ChatGPT Pro, the newest subscription tier designed for power users like researchers, engineers, and professionals working on complex tasks. It costs an impressive $200 USD/month and offers unlimited access to top-tier models, OpenAI-o1, GPT‑4o, and upcoming models like OpenAI-o3‑mini and OpenAI-o4‑mini‑high.
Also, in December 2024, OpenAI introduced OpenAI-o1’s successor model - OpenAI-o3. This model is positioned as a real revolution in the AI field, distinguished by an improved reasoning architecture and flexibility in the choice of computing resources. Depending on the task's complexity, users can configure the required level of analysis depth, choosing from Low, Medium, and High analysis options. If they need a quick answer, they can choose a low level; if the task requires serious thinking, they can choose a high level of analysis depth. OpenAI-o3 showed impressive results in tests. The model also significantly improved its capabilities in solving tasks that were previously considered exclusively the prerogative of human experts.
Until the end of January 2025, OpenAI-o3 was only available to researchers, but at the end of January 2025, OpenAI launched OpenAI-o3 mini for Pro users in ChatGPT, as well as in their API. Public access to the full version OpenAI-o3 has become available since June 10, 2025 for Pro users only. Since the OpenAI-o3-mini is a model optimized for programming, math, and science, it is noted that in the area of general knowledge, it still falls short of larger AI systems. However, in most cases, despite its smaller size, it demonstrates almost the same level of performance as such leading models as GPT-4o.
In February, 2025 OpenAI launched OpenAI Deep Research, a ChatGPT service using a version of o3 that makes comprehensive reports within 5 to 30 minutes, based on web searches.
The same month, in February, 2025, OpenAI announced the research preview of GPT-4.5. Internally nicknamed “Orion,” this model has marked a new direction for OpenAI. Instead of just chasing better logic and math skills, this model focuses on emotional intelligence and more natural, human-like conversation. It’s not just smarter, it’s more empathetic. GPT-4.5 understands tone, mood, and context better than previous models. Talking to it feels less like chatting with a robot and more like having a thoughtful conversation with a real person. Even OpenAI CEO Sam Altman said it’s the first AI that truly feels human. Some testers have even called it an “AGI moment.” The model initially became available only to Pro subscribers and in March, 2025 it also became available to ChatGPT Plus subscribers.
GPT-4.5 supports working with images in the input. It can accept graphical information, photos, diagrams, screenshots, whatever and analyze it in the context of the query. It also allows users to upload PDFs or spreadsheets to summarize or analyze them. However, GPT-4.5 currently does not support real-time voice interaction. It cannot process video files or participate in live video calls. It is not yet at the point where it can seamlessly combine video, images, and text in real time, although this will likely be a goal for future versions.
GPT-4.5 is designed to be more general-purpose than OpenAI’s narrowly focused “reasoning” models - the o-series for complex logic and STEM. In contrast, GPT-4.5 is focused on a wide range of creative and conversational tasks that require knowledge and intuition, not just step-by-step logic. It makes it suitable for a wide range of applications, from writing to programming. While still using unsupervised learning, GPT-4.5 was trained with massive data and compute power (approximately 10x more pre-training than GPT-4). It gives it even more knowledge and context. Testing has confirmed that GPT-4.5 is the “biggest and most knowledgeable” OpenAI model at the moment of its launch. When comparing models based on accuracy and hallucination the results presented by OpenAI are the following:
GPT-4.5 achieves 62.5% accuracy on fact-based questions, a notable jump from GPT-4o’s 38.2%. This makes GPT-4.5 much more reliable when it comes to factual information.
GPT 4.5 is also much less likely to make up false information (i.e., hallucinate). It scores 37.1% on hallucination rate, compared to 61.8% in GPT-4o. The low hallucination rate makes it more reliable and better suited for tasks that demand factual integrity, like research or technical support.
To make things even more confusing, while everyone’s waiting for GPT-5, the same month OpenAI quietly launched GPT-4.1, confusing the naming scheme even further. This naming can be explained as GPT-4.1 is a developer-focused API model, not a direct successor model. The GPT-4.1 family includes three versions: GPT-4.1 (flagship, only available via the API for now, not in ChatGPT itself), GPT-4.1 mini (small and fast, available in ChatGPT for all users), and GPT-4.1 nano (the smallest with the lowest latency, available in the API), each designed to cater to different performance and cost needs. OpenAI claims GPT-4.1 outperforms its predecessors, GPT-4o and GPT-4.5, in real-world coding tasks. Its 1-million-token context window, suitable for handling large codebases or documents, makes it better at writing clean, accurate code. On the SWE-bench Verified benchmark (a test of software engineering skills), GPT-4.1 solved 55% of tasks, while GPT-4o managed 33% and GPT-4.5 - 41%. That means GPT-4.1 is more reliable at reading codebases, fixing issues, and producing working code.
GPT-4.1 mini also offers a significant leap in small model performance, reducing latency by nearly half and cost by 83% compared to GPT-4o.
To make things more complex, GPT-4.1 is set to replace GPT-4.5 Preview in the API, despite the numbering suggesting otherwise. GPT-4.5’s API deprecation was announced for July 14, 2025, due to its high cost and overlap with GPT-4.1’s capabilities, making the switch in favor of GPT-4.1 practical and allowing time for developers to transition. GPT-4.5 remains available in ChatGPT for Pro users, likely until GPT-5 is launched. So, while everyone assumed that GPT-4.5 would be the hold-over, for now, GPT-4.1 seems like the next big thing.
But that’s not where the story ends for now. In April, 2025 OpenAI also started to offer OpenAI-o4 mini for fast and cost-effective performance in technical tasks. Additionally, for paid-tier ChatGPT users there's o4-mini-high in ChatGPT’s dropdown menu. The model is not comprehensively explained by OpenAI, but is supposed to offer more advanced features, including higher response accuracy, faster processing times and work well in STEM-direct and visual tasks, for example, analyzing CSV files.
What Models Are Relevant on the Current Date?
GPT 3.5 and GPT 4 are outdated models. GPT 3.5 can be tried only via the API since it is no longer available in the OpenAI browser interface. But it is practically not used now as compared to new models it has an outdated knowledge base (until September 2021) and frequent hallucinations, so its scope of application is quite limited today. On April 30, 2025, GPT‑4 was also retired from ChatGPT and fully replaced by its successor - the more versatile and energy-efficient GPT-4o. While GPT-4 is still available in the API, similar to GPT-3.5, there is no sense in using it as it's time to start upgrading to the newer generation of models.
The remaining models are relevant, with capabilities and spheres of possible applications varying by model. GPT-4o is a highly relevant general-purpose model that is available for all users, both free and paid. It is considered the latest flagship model, versatile, universal, and suitable for the majority of industries and tasks. OpenAI-o1 is relevant for complex programming, math, and science problems. Available for paid users only. OpenAI-o3 is a more capable successor to OpenAI-o1, which is currently available only to Pro users. Both are not suitable for fast responses required for everyday tasks. GPT-4.1 mini is the latest model for a fast and economical installation of technical tasks. GPT-4.5 is suitable for human-like conversations that read like those written by humans, with appropriate emotional nuances, contextual references, and a lively style. Available for paid subscribers only and expected to be substituted by GPT-4.1 or GPT-5 soon.
Bottom Line
So, over time, OpenAI's policy has changed. If earlier the bet was on universal models that were supposed to be able to do everything, now it is on specialized solutions: one is better at reasoning, the second is more convenient for writing code, and the third is ideal for everyday tasks (such as generations, etc.). For free subscribers, GPT-4o stands out as the go-to general-purpose model, excelling in text, vision, and audio tasks like brainstorming and multilingual communication. For paid subscribers, options are more vast. The o-series models, like OpenAI-o1 and OpenAI-o3, cater to complex STEM tasks, prioritizing reasoning over speed. For those seeking human-like, emotionally intelligent conversations, GPT-4.5 offers unmatched nuance. Developers and coders will find ideal GPT-4.1's 1-million-token context window suitable for handling large codebases or documents. While OpenAI's naming conventions can confuse—mixing numerical (GPT-4.1, 4.5) and omni (o1, o3) labels, each model serves distinct purposes. As we stand in June 2025, OpenAI's trajectory suggests even more tailored AI solutions ahead.
FAQ:
What is the difference between GPT-4, GPT-4o, and GPT-4.5?
GPT-4 is the original large multimodal model released in 2023. GPT-4o (“Omni”) is a faster, more versatile update with real-time data processing and better language support, available to free and paid users. GPT-4.5 focuses on emotional intelligence and natural conversations, with better understanding of tone and context, but is only available for paid subscribers.
Which GPT model should I use for general purposes?
GPT-4o is the current flagship for most users, offering a good balance of speed, accuracy, and versatility. It’s available to free and paid users and fits everyday tasks like writing, summarizing, and brainstorming.
Is GPT-3.5 or GPT-4 still useful today?
GPT-3.5 and GPT-4 are largely outdated. GPT-4 was retired from ChatGPT in April 2025 and replaced by GPT-4o. They’re still available via API but have limited use due to older knowledge bases and more hallucinations.
What does the “o” stand for in GPT-4o and related models?
The “o” stands for “Omni,” indicating a model optimized for handling a variety of inputs (text, images, audio) and better integration with real-time data.
What are OpenAI-o1 and OpenAI-o3 models?
These are specialized models designed for complex reasoning (o1, o3). They focus on math, programming, and scientific problems.
What makes GPT-4.5 special?
GPT-4.5 emphasizes emotional intelligence and human-like conversation. It understands tone and mood better than previous models, making chats feel more natural and thoughtful. It’s also more accurate and less likely to hallucinate but comes at a higher cost.
Are all GPT models available to free users?
No. Models like GPT-4o are available to free users with query limits. Advanced models like GPT-4.5, OpenAI-o1, and OpenAI-o3 require paid subscriptions (Plus or Pro) and sometimes have message caps or usage restrictions.
What is the “mini” model concept?
Mini versions (e.g., GPT-4o mini, GPT-4.1 mini) are smaller, faster, and more cost-efficient variants designed for simpler tasks or users with limited budgets. They trade some power and complexity for speed and affordability.
What can we expect from future OpenAI models?
OpenAI is moving toward specialized models for different tasks (reasoning, coding, conversation). They also aim to simplify naming and improve multimodal capabilities like real-time video and voice interaction, possibly with GPT-5.
What is the GPT-4.1 model series?
GPT-4.1 is a newer model family with three versions (standard, mini, nano) focused on improved coding skills and efficiency. It outperforms GPT-4o and GPT-4.5 in real-world software engineering tasks.
