From a matching perspective (ie having the same vendor in all positions) I would say "not that much difference". What makes the difference IMO is that the QS speakers seem to do a better job of hiding the fact that there is a speaker "over there" if you don't have room to put them a fair distance (say 8-10 feet or more) from the listening positions.
So if your room is really big you can probably put pretty much anything in and have it sound decent; if you need to sit near the surrounds then I don't think anyone has found anything better than the QS's yet.
Disclaimer - I have only listened to QS8s, not 4's.