I'll add another vote for the 4" 500.
High power rounds are not recommended for protection because they are too hard to handle and make follow-up shots more difficult. Your gun needs to big enough to allow for the nessasary power to be easily controled.
So with the 500 4" you can, very easily, drive a 450cast at 1050fps and (with this load)
it is more pleasant to shoot than the 44mag. I can shoot this load one handed with my left (weak) hand. This is an important consideration for a back-up gun since "situations" could leave you injured or otherwise unable to use one of your hands.
It penitrates incredibly well, I watched this loading shoot clear through a buffalo and shatter its shoulder on the way out.
This load in the 500 generates a TKO of 34. The 460 has to drive a 320cast +1600fps to obtain this same TKO. In order to get +1600fps out of the 2 3/4" 460 barrel you will really have to lean on it and the recoil would be extremely violent. And you won't penitrate as far with that much speed on a lighter bullet. "Just food for thought". I know there are a lot of reasons to buy the 460 (like the ability to shoot 45colt, and 454Casull in the same gun) But the best reason to buy a 460 is that is shoots SCREAMING fast with a light bullet and a long barrel, mounted with a scope it become one of the best hunting revolvers of all time...."But that is NOT what you are after for this application"
On the other side of the power spectrum for "ProtectionPower", I can drive that same 450gr. slug to 1500fps out of my 4" 500..this gives a smokin KO of 48 Of course it is hard to hit anything with both eyes shut and your head out of the way!!