only difference between the two is cost, and tone.
5" is a little more bearable for those who dont like the annoying sound of a woman whinning when your truck is warming up in the morning.
other than that you can make in exces of 700hp on a 4" no problem, and see no gains by going to a 5"
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00 Dodge Ram 2500 4x4
carter lp,Super B Special, mbrp 4", juice w/att, homemade intake, 33's, three trannies later---> almost enough power