The Gunfighter Kenai is a great holster but probably overkill for a 4-inch GP100 unless you have lower-back or hip problems from carrying a gun on the belt. I have a Kenai but use it for my big 6.5 inch N-frames.
This is my hunting setup for my 6.5 inch full underlug S&W 610 or M29.
For a 4-inch medium frame revolver I like a good belt holster and I really like a Safariland 567. It's a hybrid leather-polymer holster and for the modest price (<$40) it has been awesome. Its a leather body, so you get a swede lining with a stiff plastic reinforcements on the front, back, and top. The mounting is a very comfortable stiff rubber belt loop that hold the revolver close and stable with little wiggle when used with a good stiff belt or a paddle of similar construction can be used. There is cant adjustment and retention adjustment. There is also adjustment in the back-spine of the holster to adjust for a full, partial or no lug on your barrel.
This is my IDPA setup and I use the same holster for carrying my Model 10 around the hunting property in the summer. It rides just high enough I can comfortably sit in my truck, UTV or tractor without the muzzle hitting the seat.