For out in the woods I would go for the 6" because you can possibly nail that critter before it gets real close to ya. Like in bear or elk or even a smaller angry critter coming at you.
I guess also for older eyes the longer barrel might be a plus also. I have to say an inch longer barrel makes my marksmanship much better.
Welcome to the forum. The more you share the more you get back here at RT.
I have had both the 4" and 6" GP100's and enjoyed both of them. I sold the 6" to a buddy of mine who ended up passing away. I was supposed to have the first choice should something happen to him and his greedy sister sold it out from under me. If you want a "walk in the woods" gun, I would choose the 6". If you want a good self defense/home defense then the 4" will more than serve your purpose. It really depends on how much you are going to shoot and for what purpose you want the gun for. I keep the 4" in the house next to my recliner for home protection, however my house is an armory as I have pistols located in 5 different locations. Either way, you can't loose.
I personally would go with a 6" just to get that extra velocity outta the .357 magnum. I'm partial to long barrels
I think you'll find lots of love for the Blackhawk here. If you don't have your mind set on the GP100............
The 'walk in the woods' scenario does favor the 6" barrel. If an encounter with an aggressive crittur ensues, there is a definite advantage in the GP100's double-action. If you shoot a Blackhawk a lot, and practice double taps... the GP100 still has an edge.
You obviously need at least one of each, and probably a Super Blackhawk and a Redhawk, and and... gotta love 'em.