I got a Pro Comp 4 inch kit installed about 4 years ago - and 15,000 miles ago. It's OK, cost about $800 plus install. there were TWO options for the rear: blocks and new leafs. For some reason I got the blocks (on old stock saggin leafs)....BAD idea - there is about a 2 inch gap in the rear bump stop and leafs.
get new leafs in the rear - Pro Comp sells the 4 inchers...I'm getting them some time soon to take care of my "sag".