On March 17th, I received an email from OpenAI confirming acceptance of my GPT-4 API access, and I had access to it for a while, and now its gone. I currently have access to ChatGPT and the GPT-4 model, but its no longer an option under the “Playground” or for the API model itself.
Make sure that the “Mode” dropdown is set to “Chat”. Then select GPT-4 in the “Model” dropdown. Also, make sure your orgs settings are correct - you can read more about it in this guide