And are you sure people ask the chatbot arena the same caliber of questions they ask a single model? Wouldn't you tend to ask more difficult questions to find where one fails but the other doesn't? I'm not convinced that the average query will get a significantly better result from GPT 4. I only know that is true for more difficult queries.
Do you have access to ChatGPT? You can try it yourself (you can query GPT-4o and GPT-3.5 afterwards). The difference is large enough to be noticeable at least 80% of the time in my experience.