You can use the same model that ChatGPT uses through the API, and it isn't called 'ChatGPT' it's called 'GPT-4'. So at least officially ChatGPT refers to the webUI not the model.
I'm not sure that's true though. The first line of the ChatGPT blog post is:
> We’ve trained a model called ChatGPT which interacts in a conversational way
It really seems that GPT-3/3.5/4/etc are the generic models, and that ChatGPT is the fine tuning, a safety layer, and so on. This is also why ChatGPT and Bing don't provide the same or even remotely equivalent answers despite both running on GPT-4.
> We’ve trained a model called ChatGPT which interacts in a conversational way
It really seems that GPT-3/3.5/4/etc are the generic models, and that ChatGPT is the fine tuning, a safety layer, and so on. This is also why ChatGPT and Bing don't provide the same or even remotely equivalent answers despite both running on GPT-4.