im a massive fan of 8 injectors )rs500 style)

the reason for the 8 injectors as in the rs500 manifold is

there is an injector mounted further up the inlet track,, this helps the fuel to gather heat from the air on the ralative long journey too the combustion chamber, this heat helps atomise the fuel more! the smaller the atomised size of the fuel, the better it burns (and better emissions )
so why not just have four injectors and mount them like the secondary injectors on the rs500 manifold? well at low engine speeds the inlet air wouldnt have enough energy too carry the fuel too combustion chamber and the fuel would "drop out" of suspension

the primary injector are then used for low air speeds (revs) and high air speeds,,, but the secondary injectors are only used when airspeeds are high enough too carry the fuel so the injectors dont have too be as massive as one set of injectors as there is only a short amount of time too get all the fuel in!