To take it further, if humans had 4 arms, our heads would be a different shape, because we’d need more brain to control them. AFAIK that would mean a bigger back of head (cerebellum) and a bigger forehead (frontal lobe).
Yeah, with electronic help, like a looping device or just recording different tracks one after another, you can obviously do amazing things, but you end up lacking a lot of flexibility to quickly improvise or adjust yourself to what’s happening in the other voices.
Having said that, in an orchestra, you don’t really want most of the instruments to go wild, so simple instruments would still be a thing. Even today, there’s lots of instruments where one hand just holds the instrument.
I guess, with 4 arms, you could attempt to play two violins at the same time, though.