Lee,
the ruger GP100 is a great revolver, very durable and very accurate. The 4" barrel is just fine, the amount of velocity you will gain is not enough to make a difference at the ranges you will be taking game at. Barrel length is pretty much a matter of sight radius and balance when it comes to handguns IMO. If it feels good, and you are comfortable and proficient with it, use it. And stay away from the hollow points, they stink out of a .357 for hunting, except the fusion, out of my carbine it had a fist sized exit hole on a small doe at about 60ish yards (ill stick to 60ish atm, after a few beers and a couple more years itll be 110ish /ubbthreads/images/graemlins/tongue.gif ) didnt get a chance to use it in the pistol though so ymmv, that and the fusion isnt really a "hollow point" per se kind of in between.
For mine I used the Bsquare full length mount and loved it, then again im not really sure that there are any others /ubbthreads/images/graemlins/smile.gif.
Not sure if they make them for the 4" and you would be limited to maybe a red dot anyway due to the length of the barrel, I would get a really nice set of High vis adjustable sights and Practice practice practice, and youll be just fine /ubbthreads/images/graemlins/grinning-smiley-003.gif
Good luck and have fun.
Dave