the easiest thing to do is run the four clients for part of a day and use FAHMon to check your ppd then switch over to the SMP for half a day and check your ppd with it, then of course pick the set up that yields the highest production for you.

The other thing you might want to do is run dual SMP clients on your quad, same concept applied with SMP as with single core clients. Generally thats what a lot of quad users do if they have more than 2 GBs of ram, 2 SMP clients eat up some ram when they are running.