I had MB Quart 4" PCE's in my 91 LS4...I kept the original enclosure that housed the stock speaker...mounted the MBQ in there and applied sealant to seal up the enclosure. I'll tell ya, I wasn't satisfied with the midbass at all. So I pulled the 4" components out and threw in 6.5" MBQ Q-series components...it sure as hell made my system sound very well rounded. 4" components simply lack midbass regardless of the enclosure.
Yeah, your subwoofer is going to supply most of the bass but only up to 100hz...it would be ideal to have components capable of picking up where the sub left off (100hz) and continue with the higher frequencies. What I'm trying to say is that with 4" speakers once the sub cuts off at 100hz there is going to be a 150hz gap before the coxials will play (250hz)...the transition from frequency to frequency just isn't going to sound as smooth...150hz of "missing" frequencies will affect SQ. So basicly it's going to lack the midbass...you could try to set the crossover for 4" speakers to 100hz but they will most likely bottom out.
But yeah, we had a good run there. We have our differences but it's all good though.