four q. slang for fuck you.

the thing that you don’t have when you’re phones percentage is over 20

in GitHub, fork is a copy of a repo (repository)
so 4q sounds like "fork you" which is a simple way of saying I forked your repository.

A clean way of saying "f*ck you" is usually used by people who are not allowed to swear due to personal reasons or just want to still be "respectful"