The Short Answer
No—ChatGPT Plus does not include OpenAI API credits. Plus is a consumer subscription that improves your experience inside the ChatGPT web and mobile apps: access to flagship models in chat, faster response times during peak load, tools like advanced data analysis, image generation in-product, and priority access to new features. It is billed monthly or annually as a personal or team chat plan.
OpenAI API credits live on the OpenAI Platform, where you create API keys, set usage limits, and pay for tokens consumed by your own software. That billing account does not share a balance with ChatGPT Plus. Upgrading to Plus neither adds dollars to your developer console nor reduces API invoices. Canceling Plus does not affect API keys, and vice versa.
If you searched this question because a coupon blog claimed Plus "unlocks free GPT-4 API," you have found the correction. AI Credits documents both surfaces—see /ai-credits and /free-ai-credits for consumer angles, /api-credits and /openai-api-credits for developer angles—without blurring products that OpenAI deliberately keeps separate.