Many people get the same confusing message. My interpretation:
ChatGPT Plus provides access to API keys. We can use those keys with the v3.5 API.
Developers who were paying separately for API access have a payment history with the API. This was and is separate from ChatGPT Plus.
While we can get keys in ChatGPT Plus, note that you not have had billing for the API. ChatGPT Plus is a paid subscription to the browser interface. The API has a different payment model.
The recent announcement says "We plan to open up access to new developers by the end of this month".
So, if you have not already paid for the API with the published rate model for the API, you (and I) must wait until the end of this month.
If you have been using and paying for the API v3.5 separately from ChatGPT Plus or any other product, and you get the error, contact OpenAI.
If you wait patiently until next month, and give the company a few days rather than minutes to open the v4 API door, and you still can't use it, see if there is a general issue. If so, wait a bit more. If not, contact OpenAI. (But see below)
What is confusing is the message "gpt-4 does not exist". It should say, "your key is not authorized for gpt-4". Have patience and tolerance for our friends at OpenAI and forgive the text.
The challenge we may face: How exactly does a developer register and pay for the v4 API when it is (will be) available? I expect the site to change to allow us to pay for this product. When that happens, that's our cue that we should be able to use it. Until then, chill.
HTH