GitHub Copilot-X (Using GPT-4)

Strangely, no traction on this HN post in last 3 days.

None of Github posts explain, if GPT-4 is now being used instead of Codex for code generation cases. AFAIK, GPT-4 is not fine tunable, and GPT-4 is more general trained than for code. So I wonder how did they achieve a GPT-4 switch. Or maybe GPT-4 is only used for other use cases.

Can anyone share under the hood facts?