I only have the 2 1/2" Skyjacker kit, and I didn't do anything to my rear springs.
However, I can tell you of at least 2 members who do have the 4 1/2" SK lift on their long bed trucks - csutton7 (Chris Sutton) and Beast2B (Galen Porter). Dane might also have the 4 1/2" kit, but I think he has the 2 1/2" same as my truck... .
Try sending them a PM if they don't see this and respond.
I know a few members have said they went with all new rear springs packs from National Spring company, though I personally don't know anything about that outfit.
Tom