Microsoft's blog says:

We’re excited to announce the new Bing is running on a new, next-generation OpenAI large language model that is more powerful than ChatGPT and customized specifically for search. It takes key learnings and advancements from ChatGPT and GPT-3.5 – and it is even faster, more accurate and more capable.

Bing itself claims to be running on GPT-4:

I use a ChatGPT interface that is powered by OpenAI's GPT-4.0 technology, which is a large language model that can generate natural and conversational text.

This market resolves once more information about it has been made public and we can know for sure. If it turns out to be some intermediate model, this market resolves based on whether it's "closer" to GPT-3 and its derivatives such as Codex and ChatGPT, or "closer" to a new GPT-4 system. Resolves based on what Bing was running on at the time of market creation.