I would like to fart around with 4 4"ers, right now I am shooting 5 1/2" highback shields, getting ready to go down to the 5"ers. (shooting stickbows). There is no reason you couldnt go 3 4"
ers if your set up is tuned right. 3 5"ers is the 'standard though". Just make sure you get a clamp to match your feathers. That or get a straight jig and do an offset.