This is a great example of where quality trumps quantity every time.
I've set up some viewing rooms that, given the configuration of seating positions, having four speakers in the back was nice . . . this mainly made it possible to get more consistent coverage over all of the seats. These were all rooms "set up the short way", that is, with the screen on the long wall.
But usually, it's easier to get optimum placement with two rear channels as opposed to three or four, and as we all know, placement makes all the difference in the world. Not to mention that if you use half as many, then you have twice the budget for each one . . . which in loudspeakers can be quite handy for improving the sound.