One last thing to consider on this issue is the fact that the orientation of the blades when the arrow contacts the deer (or the test target). A 3 blade broadhead is (effectively) triangular if you look at it straight on- a 4 blade head is square. On a quartering shot, exactly how the broadhead is rotated when it hits the deer could affect how much it planes.

Using Stealth's pictures- if he rotated the heads a bit, the angle would decrease between the blades. On a 3 blade head- there are only 3 spots on the broadhead where a single blade would contact the rib cage. On a 4 blade head there is a greater chance that one blade will happen to be oriented on the "short" side- causing planing. I'm picking nits here, but it's interesting to think about. At the very least, you would have to take several shots so that you would have the broadhead strike at each angle in order to know all the planing possibilities. It's impossible, of course.

So, in theory, a two bladed head would have the least chance of having a single projecting blade strike a rib and begin to plane....