It runs both. Had seen this on the discussions on Github.

https://github.com/community/community/discussions/58059#dis...

That original note about it using both 4 and 3.5 was from this comment on November 2nd: https://github.com/microsoft/vscode-copilot-release/issues/6...

GPT-4 Turbo - the much cheaper version of 4 - was announced on November 6th. So there's a good change they've switched to that at some point in the last 6 months.

They mix it to save on compute.