Dan,
I was having a real problem getting my Thunderhead 100's to fly like my target tips and a buddy of mine had some 4 blade Muzzy's in camp this past year. I tried a couple and they shot right on the money. When we went to town all I could find were the 3 blade 100's so I picked up a 3 pack. The 3 blades did the same thing as my Thunderheads so my buddy graciously sold me some of his 4 blades.
My arrows were not tuned on an arrow spinner, I just lined up the blades with my vanes. I think the 4 blades are a little more forgiving if your arrow isn't tuned perfectly. This year I will be spinning all of my broadhead shafts, and shooting the 4 blades. There is no excuse for not spinning your hunting shafts, I have learned this the hard way.
As for penetration, I shot a nice Blacktail buck at about 12 yards that was looking directly at me, the arrow hit between the neck and the shoulder (I had held as long as I could waiting for him to turn) and was length wise through him. The broadhead went all of the way through the hind quarter but did not penetrate the hide. For a length wise shot I thought they did an excellent job. I will be shooting them from now on.
The best part is that you can pick up 3 packs at Wal-Mart for $15 during the season, and after hunting season comes to a close, they sell them off for $7. I picked up 4 packs for less than the price of a 6 pack of Thunderheads, and they come with practice blades too.