Welcome to the forum.
I would get the Qs series as they are designed to mimic the speaker arrays that a theater has and as such gives a much more diffuse/enveloping sound. As you noted the drivers are not the same in the QS4 as in the M2 or M22's, but for surround duty the slight tonal differenecs are usually not a concern and the better envelopment is worth the possible difference IMO.
If you can swing a little more money, maybe at a slightly later time to get the QS8's which have the same drivers/sound as the M22's then you will have a perfect timbre match all around
