the release today for copilot X says it's using gpt4 "With chat and terminal interfaces, support for pull requests, and early adoption of OpenAI’s GPT-4" ~ https://github.com/features/preview/copilot-x
They only positively say that they are using GPT-4 for the new pull requests feature (and one other feature that I forgot). It’s unclear what model they are using for the main copilot code generation feature. It may be that they are only using GPT-4 for features that require the larger context size.