I shoot a 4" 629 quite a bit. About 100 rds. a week for at least part of the year. Most of that shooting is with .44 Specials. (200 gr. at 900 fps.) My 6 year old daughter shoots this load so it is far from
painful.
I do though sometimes bring out a few of my magnum loads (300gr. at 1250 fps) for fun and run IDPA drills with them. I enjoy it and have no trouble getting hits (though recovery time between shots is slower than with the Spc. load). So I would say a 4" barrel is more than enough to control full-house loads.
But some of my friends who compete, and who have at least as much time behind the trigger as I do, don't care for magnums and will pass up the offer of shooting even a cylinder full.
Point is, it is so dependent on your personal physique, practice level, sensitivity, and attitude, that any advice we offer here has very little actual bearing on whether
you will enjoy it. If you think you'd like a .44 Mag, I'd go ahead and get one. Worst case scenario: you don't care for the magnums and end up loading specials. Hey, that's a pretty
GOOD "worst case!"
And the strength of a .44 Mag gun means you can start with the powder-puffs and work up loads to whatever level you might find pleasant. I have a kind of "half-magnum" load (200 gr. at 1250 fps) that a lot of my pals enjoy shooting for "big-boom-effect" because it is quite a bit hotter than the guns they're used to, but it isn't up to a level they find painful.
That kind of versatility is a great thing! Yes, it also works with a .357. But with the .357 you don't have the option of going as far up the scale.
-Sam