My first choice would be on the back wall, 5-6 ft high, and 12 ft or so apart from each other. That's quite similar to what I did in my room -- although my room is a bit bigger than your "alcove," the back of my main listening couch is also about 2 feet from the back wall.
Placing them on the side walls, again 5-6 ft high, right on either side of the listening position would also work well. If you have any doubts, do the experiments using a couple of people or ladders to temporarily hold the QS4s.
