I don't know about the API, but the ChatGPT UI GPT4 really became much worse, so much so that I had to cancel my subscription. It's not just about novelty factor. I used to store all my prompts locally, and when I compare old responses and new ones, there is a huge difference. OpenAI employee said the API model doesn't change, but were careful to not say anything about the UI. I am now waiting to get access to the API GPT4 to try it again.