Steve, I assume that "4' apart" means that they would be that far horizontally from each other(although standing vertically). If so, that arrangement doesn't take full advantage of anchoring dialog to the center of the screen, as placing them above and below the screen does. The center image would still move a little to the side with the listener, although no farther out than the M22 on that side. Mike Drew has used a similar setup with M22s to his satisfaction, however.
If you actually can position an M80 center, it would seem that one of them would be even better, but again, since you already have M22s, you could bring them out of the bedroom to see how one or two sound as a center. If you get M80s and the M22(s) as center work(s) well, you'd save money compared to adding a third M80.
