Charlie convinced me of the benefits of 4 fletch about 2 years ago. Despite the obvious 2 4" from one full length benefit, I also think that with 4 fletch, you can go with shorter/smaller feathers and still get the same stabilization. I'm using very low profile 4" nanners: they seem to stabilize as good as big feathers in 3 fletch hard helical, lose less speed, the shorter feather is a bit stiffer so hold their shape better when wet. To me, more than enough reasons to switch exclusively to 4 fletch. When I tune my arrows to where I'm happy with them, I can see no differences in any of the various 4 fletch configurations nor the orientation of any of the feathers to the shelf...they all shoot the same, so I just shoot 4x90.
And like Gator1, I think 4 fletch stabilize a little more quickly.
John, I haven't done offset but I have fletched 4x90 throughout the adjustment range of my Bitz jig...from straight to as hard helical as will fit on the shaft, both left and right wing. I can only tell just a slight bit of difference, with the harder helical settings being slightly more stabilizing. It may all be in my head, but any 4 fletch config seems to stabilize as good as hard helical 3 fletch.