I sent off my rails to Mark Gregory and went 4.5" Pro Cut on my GC4 and it's plenty tough. Almost went 4.25" but glad I went 4.5" in the end. It's on the 4-3/8" side of 4.5 though, lol.
I have played on a 4" Diamond and its downright brutal for rotation. Once it's broken in shots down the rail...