You'll hear people swear by both. Ive got the four blade. It took me a few arrows to find the perfect match, but that could have been matching the arrows to the bow. Bottom line is they are as accuarate as any other I've had.
i would lean toward the 3 blade if they both shoot good because:
1) 3 blade ferrel is slightly stonger (less cutouts for blades)
2) if hard bone is hit, 3 blades will penetrate easier
3) 3 blade is generally more readily availible
I have used both I have used the regular 3 blade 100's and the MX4's.. They both fly great out of my bow and have killed many deer.. I always got better blood with the 3 blade I do not know why.. So now I just use the 3 blade but they are both good.