I simply asked it "what are you" and it responded that it was GPT-4 based.
> I'm ChatGPT, a virtual assistant powered by artificial intelligence, specifically designed by OpenAI based on the GPT-4 model. I can help answer questions, provide explanations, generate text based on prompts, and assist with a wide range of topics. Whether you need help with information, learning something new, solving problems, or just looking for a chat, I'm here to assist!