This kit does sound very good, though I wasn't super-impressed by its fitment. In general, the diameter won't alter the sound too much, but with most of the 4" systems you get either a 4.5" or 5" exhaust tips (excluding a couple of kits that don't come with tips at all, like this system), which gives you a pretty aggressive look. However, 4" kits can result is a slight drop in low-end torque depending on the tune you're running, and 3" systems tend to have a little less drone or buzz at cruising speeds. If you want to get more specific, your best bet would be to call us directly at 1-877-578-2433.