U-Joint angles have been an issue with lift kits since William Shakespeare was still writing plays.

Just about EVERYTHING changes with a lift kit, even a 2" one. You need to install the kit, then have a good frame and/or alignment shop check everything. At the very least, it'll need realigned even with a mild lift because things gets changed and moved, including the rear axle which can shift around and the like. They need to also check for crabbing; like those "redneck" lifted Ford trucks of whom you can see one whole side when you're following it. I've seen some so bad, the guy's driving while looking out the wing window.

A 4" lift also causes problems with steering gear like control arms and tie rod angles etc. (Especially with solid front axles)

I would imagine every lift kit maker will have parts and advice as to what needs to be included, but IMHO, I think a 4" kit should just automatically include new links, tie rods, sway bar links, and whatever else to make things not be strained to limits after the kit's installed.