John,
The 'four inches' represents an antagonist's head at 50 yards, if that's all he's offering to shoot at. It also represents about an inch either side of the upper CNS from navel to noggin, where a single hit is most likely to produce an immediate effect.
On the matter of service pistol (mechanical) accuracy- any number of service pistols can hold that degree, or better. I can, and have built 1911's that will do it and run on indefinitely with good ammo. Beretta 92's will do it. Glocks will do it. You have to absolutely, positively know that your gun is zeroed with the ammo you are carrying in it. You have to know that the ammo is both effective and accurate.
The problem is that folks have been reading too much Dick Metcalf, and believing that fertilizer about "4.5 inches at 25 yards" being acceptable. As long as people settle for that level of accuracy from themselves, the gun and the ammo, they will never develop the kind of accuracy that comes as second nature under stress. If you can do it under stress, when it matters, you better change things/develop skill until you can.
It is critical to eliminate as many variables as possible because peripheral hits absolutely will not stop a determined individual. See my post in the
FBI-Miami thread for further. Sure, anybody can miss and I do on a regular basis. I also know that those misses are my error, period. I also know that if I concentrate and do it right, I can make those hits.
I personally find that comforting enough to make it worth the 'trouble' to attain the accuracy level described. I don't buy that line that 'most people' can't do it, either. I teach people to do it on a regular basis, and a surprising number of people
can- or get so close that I sure don[t want to stick my head yp with them shooting at me, 50 yards distant.