RE: 3 blade vs 4 blade
 

CG's right!
The four blade will cut a square, whereas the three blade will cut a triangle. The square, although it appears smaller, actually has a larger area than the larger triangle. Larger hole = quicker blood and air loss. The smaller profile of the four blade should fly better, too. I always avoided using them, thinking they were too small, until I found this out. Now I'm looking forward to using them this year.
Trebark

Become one with nature...then grill it.

RE: 3 blade vs 4 blade
 

All this extra size is great....as long as you aren't giving up penetration. In MY book, 2 smaller holes are better than 1 big one. That goes double if you actually have to do a blood trail. When shot from a tree stand, if you don't get an exit wound, you might not get a very good trail (on the other hand, you might). This seems to be a reason the 4 blade heads are normally quite smaller than 3 blade heads. (1" vs. 1 3/16").
That said, I'm going 1/2 way. a LITTLE smaller head (1 1/8") but still 4 blades. (just cuz the ones I'm getting don't come in 3 blades)