OpenAI’s Reasoning Models Are Changing How AI Works
Most people who use ChatGPT interact with it through the GPT-4o model — the generative, fast-responding language model that answers questions, writes content, and handles conversations naturally. But OpenAI has been quietly developing a completely different class of AI model that operates very differently: the o-series reasoning models, culminating in o4. Understanding what o4 is and how it differs from GPT-4o changes how you should think about using AI tools for serious work.
What Is a Reasoning Model?
Standard language models like GPT-4o predict the next most likely word or token based on their training. They are extremely fast and handle most tasks impressively, but they can make logical errors, skip reasoning steps, and hallucinate confidently on complex problems where systematic thinking is required. A reasoning model — specifically OpenAI’s o-series — approaches problems differently. Before generating a response, the model runs an internal “chain of thought” process that works through the problem step by step, checking its reasoning, identifying errors, and revising its approach before outputting an answer. This process is computationally expensive and slower than direct generation, but it produces dramatically more accurate results on problems that require multi-step logical reasoning.
o4 vs GPT-4o: The Key Differences
The most important differences between o4 and GPT-4o are response time, accuracy on reasoning tasks, and appropriate use cases. GPT-4o responds in 1-3 seconds and is excellent for conversational tasks, writing, summaries, and fast information lookup. o4 takes 15-60 seconds (sometimes longer for very complex problems) because it is actively working through its chain of thought before responding. In exchange for this wait time, o4 is significantly more accurate on mathematical proofs, multi-step coding problems, logical puzzles, scientific calculations, and any task where systematic reasoning is required. On OpenAI’s own internal benchmarks, o4 correctly solves 87% of competition mathematics problems where GPT-4o achieves approximately 52%.
o4 Mini: Accessible Reasoning for Everyday Use
Alongside the full o4 model, OpenAI released o4 Mini — a smaller, faster, and cheaper-to-run reasoning model that brings the chain-of-thought approach to tasks that do not require the full power of o4. o4 Mini responds in 5-15 seconds, costs significantly less per API call than o4, and is accessible even in ChatGPT’s free tier (with usage limits). For most everyday reasoning tasks — debugging code, checking logic in a written argument, solving word problems — o4 Mini delivers most of o4’s accuracy advantage at a fraction of the computational cost.
When to Use o4 vs GPT-4o
Choosing between these models depends on your specific task. Use GPT-4o for: writing emails and articles, summarising documents, answering factual questions, generating creative content, image analysis, and natural conversation. Use o4 for: solving complex mathematical problems, verifying the logic in a legal argument, debugging multi-file code architecture issues, analyzing scientific data, working through multi-step financial calculations, or any task where you are concerned about accuracy and willing to wait for a more careful answer. Use o4 Mini for: coding assistance, checking the correctness of a written argument, solving math problems at high school and undergraduate level, and any task where you want better than GPT-4o accuracy but faster than full o4 response times.
Access on Mobile: ChatGPT App for Android
Both o4 and o4 Mini are accessible through the ChatGPT app for Android (free on Google Play). ChatGPT Plus subscribers can access both models freely, while free-tier users get limited o4 Mini access. To switch models in the ChatGPT app, tap the model name shown at the top of a new conversation and select from the dropdown list. Both reasoning models also support vision — you can attach a photo of a math problem, a diagram, or a code screenshot and o4 will reason over it systematically rather than just describing what it sees. For more AI tool guides and feature comparisons, visit Android People.