I agree with ChristopherG, the S&W 686 4" is IMHO THE best revolver ever. Of course, I hate single-actions ... they're meant to "roll up" when fired ... and loading/unloading is tedious compared to a DA revo. Also, I would suggest that with the 686 available, you need not consider the Ruger GP100.
For home defense, you need only concern yourself with feel and effectiveness. No need to go to a J-frame ... in an effective caliber, it's likely to be more than wife would care for.
I would definately not go shorter than a 3" bbl and definately not longer than a 5" bbl.
The only issue I would be concerned with is that your wife might not be able to get used to .357mag in the 686 and I see no need to load with .38spl+P for home defense (although, the ability to shoot both in the 686 would help your wife work her way up to .357mag).
An excellent alternative would be the 4" 625 ... although, you might have trouble finding "mild" .45acp ammo to help your wife get used to it.
In the end, I believe any woman can get used to .357mag in the 4" 686 ... it's just not bad ... but, a lot more than a .22 nonetheless.
If S&W offered a 696 (.44spl) in a 4", I'd say that would be good too. But, I feel the 625 or 686 would be a better choice.
I realize that you'd like to get a revo and I think that's great. But, if it's home defense and recoil and your wife you're concerned about, I would like to suggest a Glock 35 .40sw. Recoil is pretty much non-existent. 10+1 capacity. Very beginner-friendly trigger. And, just as safe as a revo (same safety concept ... don't pull the trigger unless you intend to shoot). If you can, I would highly recommend that you AND your wife rent a few guns ... including a Glock 35 and while firing, only fire the revo's in DA only and compare your targets. If the Glock 35 target doesn't impress you, then get a revo. If the grip of the Glock 35 does not seem to fit, check out a Glock 23 ... the finger grooves are closer together (I have no clue if that would make much of a difference). If the Glock 23 fits better, I'd recommend the Glock 23C for quicker follow-up shots. The Glock 23C might have ever-so-slightly more recoil than the 35 and a louder "bark", but, it's still very easy on the hands.
Good luck.