Following up - I did sign up for a ChatGPT Plus membership, and it does give me access to the GPT-4 model. However:
a) the token length limit for this GPT-4 model is still the same as for the GPT-3.5 (4097 tokens, rather than 8k or 32k). I’m disappointed with that.
b) Access is only through the ChatGPT interface. Neither the API nor the Playground can be used with the ChatGPT Plus membership.
Sure you can have GPT-4 enabled on the chat.openai.com as a benefit of the Plus subscription but that doesn’t mean you have access to the API. ChatGPT is a separate product from the API