the only problem with the eaton m90 is the manifolds are different so you have to fabricate something to hold the supercharger and the way they are shaped and mount its a pita imo. all thou like pud said if you can fabricate and want to go that route the m90 would be a good choice. you can even get the thing ported to flow more. and on the supercoupe they run 12 psi stock so that should be plenty for your 4.0. i belive that one of the bigger companies makes one. if im not mistaken vortech offers a kit if your not into looking to fab anything yourself. i just googled for 4.0 ranger superchager and it was vortech from summit that has the kit.