Shock relocators will give a better ride than just using a shorter shock, at 4" drop the shock is at to flat a angle with the ground to perform as designed and the shock relocators allows them to be at the proper angle to function correctly. In regards to needing the adjustable track arm bar you are right on the borderline, the only way to know positivly is install the rear 4" springs on truck with all of body installed so full weight is on suspension then measure the rearend to see if its centered, I find 70% of the time or more that the adjustable bar is required to center rearend properly under the frame on a 4" rear drop. You'll find everything you need here
http://www.gmcpauls.com/lowering_springs_&_parts.htm
GMCPaul