The cost is pretty much the same? Any input would be greatly appreciated. My room is 18x15

You must consider what your mains and center speakers are. Your surrounds should match them. Note that QS2 has 3.5" woofer while QS4 has 4.5" the sound of one does not match the other.

Hi,
My current set up is:

Pioneer VSX/D810S
Vp 100 center
M3ti
QS2

The wall on the back of the room is about 12 feet long.
One side of the room is completely open, so i have to put the surround speakers on the back wall. I think 3 QS4 would be to much for the rear wall. But if 2 QS4 sound better the 3 QS2 i would concider getting them.

Thanks

I assume you have a wide screen tv, therefore your viewing position is pretty close to the back wall and you only have the front wall, left side, and back wall. With this configuration you are forced to have 5.1 set up.

Having this in mind, you are better off with 2 QS4's mounted on the back wall. I do not see the advantage of placing a third surround when the distance between the 2 surrounds is less than 12 feet and you are already seated pretty close to the back wall.

By the way do not forget a sub. Your set up will sound feeble without one.