There's no "ChatGPT API". ChatGPT is a webapp. It lets you select between 3.5-turbo and 4 models in its UI.
There's an OpenAI API, which recently added chat interactions and suitable models. There, too, you can select 3.5-turbo and 4. AFAIK, the 4 served by the API is the same 4 used by the ChatGPT web app.