A Carb/Throttle body spacer works best on a Carb or Throttle body injected engine, giving the air and fuel more time to atomize (mix). The 4.0 is a direct injection engine so all the spacer is going to do is give more room for the air coming into your engine to travel. Some people have reported a better throttle response and some a whistling noise. Neither of which I feel is worth the money.