I use GPT-4 for code completion all the time! There are many Neovim extensions[1][2][3] (and I'm sure there are many VSCode extensions) which call the GPT-4 API directly for code completion. I'm pretty sure the only reason that Microsoft might avoid using GPT-4 for Copilot is cost.
[1] https://github.com/cosmojg/nvim-magic
[2] https://github.com/dpayne/CodeGPT.nvim
[3] https://github.com/aduros/ai.vim