First, there's been tons of comments on these, so I recommend firing up the search engine.
Short form: great gun, tough as nails, easy to work on, good accuracy, trigger needs minor help sometimes (in a cheap, DIY fashion) or just dry-fire the snot out of it (which won't hurt it any). GP100s are often used as 357Mag ammo development mules by Cor-Bon and others - for HOT loads.
The closest equivelent is the S&W L-frames (586/686). Of the two, in my opinion the GP100 is the better gun. The out-of-the-box trigger won't be quite as nice, but the GP was meant to be field-stripped almost without tools and help is just a bit of internal buffing and spring kit ($17) away.
(When you take the grips off with a single screw, you're done with the screwdriver. Buried under one grip panel is a takedown tool for everything else. There's no frame screws to booger up. A lot of people picking between GP and L-Frame get hung up on the usually minor trigger difference, and stop there to go S&W. This is a mistake, in my opinion.)
For personal defense the 4" is fine. It's not half bad for target work either, although there's going to be some small advantages to the 6"ers. Some people like heavier barrels, some lighter...get what you want for the shroud type.
The big advantage to the 6" tubes is for critter work - you can get certain super-hot hunting loads moving at enough velocity to make the minimum power level for legal deer hunting in most states (158grain @ 1,500fps or that sort of horsepower level). This also lets you send 180 or 200grain solids with enough "heat" for boar hunting or *black* bear defense...basically, a 6" barrel GP100 can kill just about anything in the lower 48, unless there's still a few Griz around here
. It's a bit on the light side for the biggest boar or black bear, but still doable.
The balance on the 6" short-shroud guns are amazing - they handle like a 4"er. If that's your thing, definately give one a "feel test".