I'm fairly sure this is because of the longer context window they've probably enabled for GPT4 on ChatGPT, I'm seeing the same thing. When you use the longest

I'm fairly sure this is because of the longer context window they've probably enabled for GPT4 on ChatGPT, I'm seeing the same thing. When you use the longest context window, it tends to place the most focus on the first and last message, for whatever reason. So if you instead of continuing the conversation, you start it from the beginning but with the additional information gained from the previous one in the initial message, it does a lot better.