The fire wall has a seam where two pieces come together. When you go to a 4in dp there is not enough space to fit the pipe without it resting on the seam. If the dp is touching anything you will get a vibration that is felt through the floor. I have a Hog system that I installed and I had to use a hydrolic tool called a port-o-power to flatten the seam for clearence. Go to www.hogperformance.com and watch the video clip on the install. I dont know if all 4in dp are like this but it does get real tight in there.