4.25" are perfect. The only reason I know this is because I had to cut 3/4" off of mine.

I went out on about an 80 mile ride this morning. It took a little getting used to the thicker grip. They don't move (extra hold, grape flavored hairspray FTW!). They did dampen the vibes somewhat. My left hand didn't go to sleep at all and it took a lot longer for the thumb on my right to go numb, so that's good.
I think I need to get a set of Murph's risers next, as I find myself leaning on my wrists. It takes conscience effort to straighten my back and squeeze the tank. The cycle repeats every few minutes.