13 psi will not need a fifth injector

in SA before local tuners reversed engineered the Ausie Microtech and built the Domingos(now Gotech) standalone system , we used a hop switch(adjustable pressure switch) wired to a microfueler with an injector plumbed into the intake

at a set pressure , the hop switch closes , activates the microfueler which pulses the injector. The microfuelers pulse duration is adjustable

arcane way of dumping fuel into your motor though