You are asking the unanswerable, because recoil is so subjective. From what you've said, I'd get the 4".

I owned a 6" for a short while and found it to be muzzle-heavy, whereas the 4" is balanced much better. (I'm betting this is why your wife prefers it.)

As far as recoil, I find 357s to have a sharper, straight-back recoil with "expected" but not excessive muzzle flip. Is the 6" better in this regard? Yes, but if she can't hold it at arm's length well enough to line up the sights that won't matter either...

Overall, I believe you're over-worrying this. Get the 4" and shoot 38 specials out of it until your both good shots with it before ever using 357 ammo. The 357s aren't that bad, but get your confidence and abilities up first, so any sensitivity you do have is minimized by the skills being built-in by then...