Congrats on your purchase, nice looking (sounding) setup!
I have the Epic Master system (It's on the products page if you're not familiar with it) in a setup that's similar to yours (large open space to the right of my right front). I'm not running a 6th channel in my 20 X 12 room, and my QS4s do an excellent job of dispersing sound (couldn't imagine being any happier with them). They're set up on the Axiom stands (height of approx 48"), so they're not as high as your wall mounts. I'd think you'll do just fine, especially with another QS4 in the back to "fill in" the sound. As others have done, you can enlist a friend to help hold/position the surrounds as you tweak placement, but I think you'll find that the QS4 design is pretty forgiving for those of us who aren't blessed with an ideal viewing environment.
good luck!
lando
