My .357 GP-100 has a 4" barrel. That's how Ruger made them in the '90s.
Now, they make them 4.2" instead of 4" which I believe is to comply with Canadian gun laws on handgun barrel length.
I always wondered about the barrel length on Marlin rifles and assumed (I know, when you ASSuME ...) that it was for ballistic reasons, but the BALANCE of the rifle makes a lot of sense (which, i think, is why some of the longer barrels have the shorter magazine tube).
And then there is the cosmetic factor of the Cowboy models.
I really like the 16" Trappers (which are actually 16 & 1/4" nowadays to avoid any question of being too short for legal issues) - when they get that short, balance is less of an issue for me because the 'handiness' makes up for it.