So I found a great deal on an enclosed 32x8.5' flatnose gooseneck. I'm headed south to pick it up next weekend. My truck is a shortbed, just put a B&W hitch in it. When I first measured everything it looked like I could jack knife the trailer if I had a 16" extended coupler. Well the 16" came and it doesn't quite do it. If I bought B&Ws 4" extended ball mount for use with the 16" coupler, I could jack kinfe the trailer and easily back it into my driveway. I do have airbags to stabilize the truck but, it seems like there will be an awful lot of leverage at work with the 4+16 combo. Any thoughts?

You don't mention any weights. Your gasser is limited to 16,000# GCWR.

The trailer should be around 9000lbs when loaded, truck is about 6250lbs ready to roll, including me. I have no idea what the pin weight is going to come out at. My biggest concern is wear and tear on the B&W hitch itself when combining the 4" ball mount with the 16" offset coupler.

the ball mount is attached to the truc, and the coupler is attached to the trailer right?
if so the hitch will be effected by the ball mount, and not the coupler. the trailer frame will be effected by the coupler, and not the ball mount. you are stressing both truck and trailer, but (hopefully) not overstressing either.