We barely have a single playable race outside the player hand book, and it is there solely because it is so closely tied to the narrative of the game. A homebrew race has pretty much no chance to be officially made.
But don't despair, people have already modded races into the game (see
https://www.nexusmods.com/baldursgate3/mods/215). A race with four arm would probably be very hard to implement though.
