I've run rips 4inch on both my 24v and my old 12v. On the 12valve I originally ran the stock exhaust straight piped, then went to a 4inch straght pipe with the stock down pipe, and then to rips kit because the drone was too much. Rips kit with the muffler was awesome. Lets just say this, two days before I picked up my 24v. I had rips 4inch kit sitting in my shop waiting. I love the sound and the price is great. With that said, I'm thinking of switching to the 5in straight. My buddy ran that on his truck which was set up similar to mine, and that truck sounded like nothing I've ever heard before, and noise in the cab wasn't bad either. The 5inch is a bit louder though. I have no hesitation in recommending his 4inch kit.
Sean