4"
The gun comes with both compensators.
There is more than aesthetics here, as the manual calls for the shorter compensator to be used for JACKETED bullets and the longer compensator for lead cast bullets (without the top vent holes just under the sides of the front sight).
When I first got the gun, I shot some 440 grain hard-nosed lead bullets using the wrong compensator. It shot fine, however i found some atomized lead on the sides of my front sight which was pretty difficult to remove. So, I surmised that the lead one is designed that way for a reason.
Also, I found that the compensator loosens. S&W replaced the Allen bolt that secures it, and when I asked if I should put Loctite on the bolt, they told me no.
I used red Loctite on the bolt ANYWAY as the bolt would loosen after a shot or two, which in turn loosened the compensator just enough - so that each shot was essentially 'hammering' the moving compensator against the bolt which caused the first bolt's ridges or fins, (or whatever you call them) to be noticeably wearing off, in fact, wearing very quickly!
Cleaned everything up with Rubbing Alcohol and One drop Red Loctite on the bolt threads at the end and I have not had a loose compensator going on over a year now. I realize I will need some heat, should I decide to remove the compensator and bolt, however that is fine by me.