Since you already have QS8's for the sides, you would be best to use QS8's for your rears too. The 8's, imo, offer much more speaker for not much more $$ compared to the 4's, but that's entirely up to you. Your room is similar in size to my room, I don't think I'd go with one of the "M" speakers as rears, you are too close to them imo.

Half of communication is listening. You can't listen with your mouth.