something to consider, at 4 inches you'll either want to notch the frame, or at the very least put a smaller bumpstop in the rear. here's my truck with a 3/4. coils in front and shackles and hangers in back. Both bumpstops are bottomed out (front and back)
But this won't actually change the suspension. Just lowers it. If you need a smoother ride you could look into what type of tires you have (are they passenger class, or heavy duty?)
If you wanted to spend some bucks, you could put in some air bags, or convert the back end to a 4-link coilover setup or something.