I agree that everyone is different and it really depends on your body shape and size to determine if ccw'ing a 4 inch GP100 will work.
I have CCW'd my 4.2" Match Champion before pretty successfully (I think, anyways).
I would recommend considering OWB carry for it, because the cylinder is pretty beefy and makes it difficult to carry IWB.
Cover garment is ESSENTIAL. A T-shirt may not be enough...you may need to use a blazer, a button-up shirt, or a jacket, but this sort of OWB carry really depends on the cover garment.
I use the safariland 568 paddle holster for the GP100. I think it's very comfortable, and holds the revolver well. That particular holster holds on to the belt very well also, and doesn't disassemble. I also find it very well made. Very pleased with that holster.
I usually will tuck the tip into my front pocket (wearing jeans), and use a good cover garment. With a jacket, it conceals very well. Even if I forget and lift my arm, the part of the holster exposed under the jacket is black, so it usually hides well in the shadow below my jacket. If someone sees that part of the holster, I think it will look more like a cell phone sticking out of my pocket, or maybe part of my jeans, as opposed to the holster.
A good belt is essential also, of course.
If you are interested in IWB carry, the SimplyRugged Holsters I've seen are great for this as well, and can easily do the trick. You will need a pretty big belt, but from what I've seen, the SimplyRugged holsters are comfortable and can conceal well.
Another option for IWB is the Aliengear cloaktuck 3.0 holster -- specifically the one for the k-frame smith and wesson. If you get that holster, and loosen up the shell a little with the provided spacers, it will fit your GP100 pretty well. I tested that myself, and it does work. That being said, I did have some fit-finish issues with the AlienGear holster (grommets came out, some things fell apart), so I ended up sending it back. But, if you want to go that way, it DOES fit.
You may want to change the grip on your GP100 as well -- some grips really stick out and make it hard to conceal. The Eagle Grip Secret Service may be the smallest grips possible for the GP100, and I find I really like them. I don't have problems gripping the gun, and it conceals very well. Looks really good too. =)
In terms of other guns (if that is an option), the SP101 (my preference is 3" barrel) conceals IWB much better than the GP100 -- here, the 5-round cylinder really shines (as opposed to a full-size 6-round cylinder) by decreasing width and making it much, much easier to carry IWB. I use a DeSantis Pocket-Tuk with the SP101. The end of the holster is open, so even though it was made for the 2.25" barrel, the 3" fits just fine -- just sticks out the end a little. Usually it's more comfortable wearing Boxers with it rather than briefs -- the tip of the barrel is pretty cold and a little uncomfortable when sitting.
I find that the Pachmayr Diamond Pro grips are inexpensive, fit my hand very well, points great (for me), and absorbs recoil amazingly. They are bigger than some other options however, such as the Uncle Mike's grip or the stock grip.
I also have a friend who uses the SimplyRugged pancake with the SP101...he loves it and says it carries very, very well.
One last option (as has been mentioned): consider a six series gun (Security, Speed or Service six). They aren't supported by Ruger anymore (officially, anyways), but they are more K-frame sized and will likely conceal much more easily IWB or OWB than the GP100. Slightly thinner cylinder is to credit for that.
But as far as carrying the GP100, it is definitely doable and even comfortable (well, except for the weight). A very, very solid choice, in my opinion.
P.S. One more option: can consider the Galco Wheelgunner as OWB carry. Cover garment recommendations apply. Only problem with the wheelgunner: you will probably really need to use the hammer retention strap with this holster, or if you bend over, the GP100 might fall right out. No retention at all except for the hammer retention strap, and that strap (at least for me) takes some doing to take off before I draw...it's not a natural movement that can be incorporated into a drawstroke (at least for me).