With your couch up against the back wall you are somewhat limited to what you can do but still in good shape for 5.1 I wouldn't bother with a rear center channel. It will end up directly over your head and probably be more distracting than actually presenting 'rear' oriented sounds.

Depending on your room layout, the QS4s should be relatively to each side of the couch. Again, due to couch positioning, you don't want them exactly in the rear corners so a few feet to the front will not hurt. I understand the QS lineup to be very forgiving to this because of the way they can direct & reflect sound backwards against your rear wall. They should also be a few feet higher than your listening height. Just make sure they are not tight against the ceiling as they have a driver that points upwards and it needs a foot or two of room to breath.

Speaker stands are also an option for the QS4's to place them to either side of your couch if the walls present problems.

With great power comes Awesome irresponsibility.