EdD

it very much depends on the size of your room. If you have a very large room, you'd want the extra volume of the QS - 8's so any rear sound effects don't sound hollow and far away. In a mid sized room, the QS - 4's would be very good. If you want to listen real loud, however, then you should get the QS - 8's.
I have heard putting center channel speakers in the back also sounds good. Maybe for the center rear speaker, you could put a vp150