LazyJ, this is a bit of a backwards question.
There is no exact cutoff for a room size before one goes to a tower specfically over a bookshelf or for the QS8 over a QS4 or the VP100 vs. the VP150.
The Axiom speaker/HT guide does however already provide recommendations based on room size, however this is not always the single driving criteria.
You need to factor in budget, preferred sound type, etc.
For us on the forums though, your room size and layout IS an important piece of info before attempting to give advice on speaker choices. It is not a 'bias' of data. It is REQUIRED data to make an informed opinion.
Since you are set on the M22s and not a larger tower, i'm assuming your room is not overly large, or at least your listening area is a reasonable size. In such case, any of those options you have will work.
Four QS4s will give you more speakers for a surround setup. The larger QS8s will provide cleaner sound at higher volumes in larger rooms but will give you 2 less speakers in which to create a sound field.
Which version would sound "better" is solely up to you.
