A 4" gun with a .004 gap and and real smooth rifling may well get better velocity than a 6: with a larger gap and somewhat rougher rifling. One barrel is .3575 and the other is exactly .357. One load uses fast burning Bullseye and the other slow burning 2400 and there will be a bit less difference.

But, here is what I feel about that. The sighting radius will have more effect than the velocity. The effect on anything struck with a bullet going 100fps slower will be minimum and and anything poorly struck by either won't show much difference either. Your bullet being just 1" closer to fatal areas will have more effect.

That said I would rather be up against against a mediocre shot with a 6" gun than a good one with a 3" at 25 yards or more.

Which brings up another point. The ballistics of a handgun bullet. The drop from a bullet going 100fps less at 100 yards is negligible when compared to group size 99% can achieve at that range. At closer ranges the trajectory differences are even negligeble

I used to be a big fan of 6" guns and once built a Super Blackhawk 44 mag with a 14" barrel. Now, I mostly like to carry guns 4" or under. Walking around a 6" is ok on your hip, but sitting down, driving etc it is more problematic. Not terrible, just not as easy for what I think you gain.