One Flo-jet G57 will easily handle a 4-bay system. It's very rare that all 4 bays will be using FB at the same time. Regardless, my busy 8-bay uses one Flo-jet G57.
A couple of 4-bay Kip solenoid blocks, a couple of mini air pressure regulators, a handful of push-to-connect fittings, 2 spools...