So, this raises several questions!
Is there a way Mathematica would now I have a ChatGPT Plus Subscription with a wider rate limit?
Does the Mathematica plug-in has also rate limits?
I believe that the permissions (e.g. gpt-4 enablement, etc… and rate limits are just attached to your OpenAI API key which itself is for your particular account at OpenAI. Mathematica, I believe, is not acting as a gatekeeper other than to put in your OpenAI API key into a secure location to use.
Others at Wolfram can confirm or advise (I work at wolfram but am not currently working on the parts that have to do with OpenAI permissions – though I am working on LLMs n various ways: so I am just “guessing” on this)