Ok, slightly different approach here...nonetheless I agree with a 4" barrell.
I have a 4" Smith 625 and a 6" GP100. I often with I had bought the GP100 in 4", but that is 10 years after the fact.
Reality: I'm damn near blind as a bat. 20 yards is pretty much my maximum effective range with a handgun using iron sights. Two hand unsupported is around 6" groups, from a rest it's more like 4" groups. I just can't group any better with my eyes. To do better I need a scope. So the 6" barrell and longer sight radius is really pointless to me. I shoot my 4" 625 just as well because (get this!) the Shorter sight radius is actually easier on my eyes!
So, ask yourself the question about what distances you are shooting at. IF you are talking about hunting, would you need a scope? If so, then get the 6" barrell, scope it with a nice 2x or 4x pistol scope and try to find someone who makes a holster for it.
IF you are talking about plinking, target shooting to say 50', or as a back-up gun when hunting (back up to your .308, etc.) then 4" is absolutely the better way to go.
I am now getting closer and closer to just having a weaver rail mounted on my GP100. IF I want to shoot at the indoor 50' range, then I can always just drop a cheap red-dot on it and keep right on truckin'...