1 | Why So Many Models?
OpenAI now ships two parallel families:
| Family | Core Goal | Optimised For |
| GPT-series | Breadth – huge unsupervised pre-training for general knowledge + fluent text | Creative writing, multilingual chat, knowledge retrieval, vision & audio (in 4o) |
| o-series | Depth – explicit planning & tool-use reasoning | Multi-step maths, coding, data analysis, autonomous workflows |
Rather than “new replaces old,” models are tuned for different budgets, latencies, and reasoning needs, giving builders a menu rather than a single “latest.” OpenAIOpenAI Community
2 | Timeline & Genealogy
Solid arrows mark official releases; dotted lines (not shown) represent internal iterations.
3 | Deep Dive: GPT-3.5 → GPT-4 Turbo
GPT-3.5-Turbo – the workhorse
Launched: Nov 2022
Context: 16K
API cost: $0.002 in / $0.006 out per 1 k tokens OpenAI
Best for: prototypes, high-volume chatbots, first-draft content when budget trumps accuracy.
GPT-4 (legacy 8K)
Launched: Mar 2023 – the first widely available multimodal GPT (image inputs).
Context: 8K (32K retired).
API cost: $0.03 in / $0.06 out.
Best for: regulated workflows already audited on v4; still valued for determinism.
GPT-4 Turbo
Launched: DevDay Nov 2023
Key upgrade: 128K context + 3× cheaper than GPT-4 ( $0.01 in / $0.03 out ) OpenAI
Best for: long-form document QA, contract analysis, codebase chat.
4 | The Multimodal Leap — GPT-4o (“omni”)
| Feature | GPT-4 Turbo | GPT-4o |
| Text & images | ✔︎ | ✔︎ |
| Real-time audio I/O | – | ✔︎ |
| Speed vs 4 Turbo | baseline | ≈2 × faster |
| Cost | $0.01 / $0.03 | $0.005 / $0.015 OpenAI |
| Context window | 128K | 128K |
4o’s single model handles voice, vision, and text in one feed-forward pass, enabling live, near-human video demos OpenAI.
Use it when: you want the richest UX (voice-chat, screenshot Q&A) at mid-range cost.
5 | Beyond Knowledge — The o-Series
What makes an “o” model different?
- Longer internal “thought” budget – the model learns to deliberate.
- Native tool use – in ChatGPT it can autonomously open Python or the web.
- Vision-reasoning baked in.
o3
Launched: 16 Apr 2025
Profile: highest reasoning/coding scores in OpenAI’s public suite, tuned for autonomy.
Price: $0.01 / $0.04? (API tiers mirror Turbo) – official card: $10 / $40 per million tokens OpenAI Community
Sweet spot: data-science notebooks, multi-step coding help, advanced tutoring.
Why o3 stands out
Unlike the GPT family, o3 can act while it thinks. During its chain-of-thought it decides when extra evidence is needed, then autonomously invokes any ChatGPT tool: live web search, Python execution, file analysis, or image generation. It can pull public data, run a script, plot a chart, and explain the result—typically in under a minute—while performing light self-fact-checking to curb hallucinations. If your workflow demands data-backed answers that blend text, numbers, and visuals, o3 is currently the most capable publicly available model.
o4-mini
A cost-efficient sibling (≈ 30 % of o3’s price) for “good-enough” multi-step tasks OpenAI.
Sweet spot: batch code review, lightweight autonomous agents.
o-mini vs GPT-3.5
Although some folks substitute o-mini for GPT-3.5, remember that o-mini is optimised for reasoning, not chit-chat; its writing style is plainer and sometimes slower.
6 | What About GPT-4.5 Preview?
4.5 is a public canary build:
- More emotional nuance – marketing copy reads less “AI-ish.”
- Small factual & code-gen gains over 4 Turbo, but still costlier (>$0.008 in).
- Available only in ChatGPT Plus / Team for feedback; not yet a stable API model.
Use it to test edge cases in creative or brand-tone-sensitive content, but don’t lock production flows until GA.
7 | Decision Matrix
| “I care most about…” | Pick this first | Why |
| Lowest cost / scale | GPT-3.5-Turbo | 5 × cheaper than any GPT-4 tier. |
| Realtime voice or vision UX | GPT-4o | Native audio + faster latency. |
| >100 K-token workspace | GPT-4 Turbo or 4o | Same 128K, pick 4o if audio/vision needed. |
| Deep reasoning / tool calls | o3 | Premier chain-of-thought. |
| Creative polish / subtle tone | GPT-4.5 preview | Richer stylistic control. |
| Regulated, validated compliance | GPT-4 (legacy) | Deterministic sampling. |
8 | Cost & Performance Benchmarks
All prices USD / 1 k tokens, Apr 2025.
| Model | In | Out | Relative speed* |
| GPT-3.5-Turbo | $0.002 | $0.006 | ★★★★☆ |
| GPT-4 | $0.03 | $0.06 | ★★☆☆☆ |
| GPT-4 Turbo | $0.01 | $0.03 | ★★★☆☆ |
| GPT-4o | $0.005 | $0.015 | ★★★★★ |
| GPT-4.5 prev. | $0.008 | $0.024 | ★★★★☆ |
| o3 | $0.01 | $0.04 | ★★★☆☆ |
| o4-mini | $0.003 | $0.012 | ★★★★☆ |
*Speed ranking combines latency & tokens-per-second averages from OpenAI’s April 2025 dashboard.
9 | Prompting Tips by Model
| Model | Tip #1 | Tip #2 |
| GPT-3.5 | Be explicit—fewer hidden assumptions. | Break long tasks into numbered steps. |
| GPT-4 Turbo | Use system messages to lock tone for long docs. | Exploit 128K to paste entire manuals. |
| GPT-4o | Include small image snippets to ground context; for voice, punctuate clearly. | Use “speak as” in system role for voice persona. |
| GPT-4.5 | Leverage style-transfer: e.g. “rewrite with empathetic tone for Taiwanese tech readers.” | Provide brand lexicon to push creative boundaries safely. |
| o3 / o4-mini | Allow the model to plan: “Think step-by-step before answering.” | Give structured JSON schema for function-calling—reduces hallucination. |
10 | Looking Ahead
OpenAI’s public roadmap signals:
- Gradient of effort knobs – o-series already offers low/med/high reasoning passes; expect similar controls in GPT-series for latency-sensitive apps.
- More multimodal fusion – 4o’s single-pass audio-vision will likely cascade into 4.5+ and o5.
- Native agents – ChatGPT’s tool-calling hints at sandboxed “micro-agents” executing on-device for privacy.
11 | Conclusion
- Not a ladder, but a toolbox. Each model trades cost, speed, modality, and reasoning depth differently.
- Prototype, benchmark, iterate. No doc (even this one!) beats a 100-message pilot with your dataset.
- Stay agile. Prices have dropped >80 % since GPT-4’s debut; workflows locked to one tier risk overpaying.
TL;DR
| Budget | Go-to | Upgrade path |
| 💸 Shoestring | GPT-3.5 | Add o4-mini for tricky code |
| 💼 SMB app | GPT-4o | Slot in o3 for analytics |
| 🏢 Enterprise | GPT-4 Turbo + o3 | Test 4.5 for polished CX |
May this guide help you navigate the expanding model landscape and pinpoint the tool that best elevates your AI projects.
