I agree with the last two posts above mine. I would add that you should have a long arm available for HD. Why debate one vs the other when you can have both? I have a 12 gauge shotgun and a double-stack .45ACP both within arms reach (as well as my carry gun, so I guess that's three).
Primary plan is to call 911 and cover the door with the shotgun. Secondary plan is to take the .45 through the house. However, highly trained professionals have a fairly low success rate at clearing houses with bad guys in unknown locations, even in teams. Its just too easy for someone hiding to shoot you before you can find and shoot them. Clearing the house with any gun is a very last resort. This is why the long arm is superior; because if you are using the proper tactics (hiding in your room and letting the police do the room clearing), its disadvantages are minimized.
One thing that hasn't been brought up yet is a light. Most revolvers, including the GP100, do not have a way of mounting a light. That's ok, if you have a suitable flashlight nearby, and are trained in its use. Personally, I find the added training burden of working with an offhand light, combined with the accuracy reduction of shooting one-handed, to not be worth it, so I run weapon-mounted lights on both my HD guns.
It is absolutely essential that you have a light worked into your HD plans. Don't count on the lights working, or being able to get to the lights, or even wanting to give both you and the bad guys the advantage of light. You need to have a portable light and know how to use it.
So with that stuff out of the way, the 6" is not substantially better or worse than the 4" for HD. There are much bigger factors out there. I will reference my signature for this one.