I've always wondered about this, on some trucks, it seems that 6" isn't enough to clear 33" tires, and on others, you can get away with 35's without any lift at all!
I had a half ton truck, that fit 33's with no lift, and then I bought some 36" Buckshot mudders, stuck them on, and it drove fine, but any flex at all while turning, and I would rub, so I put a 3" body lift on it.. Buddy of mine ran a 3/4 ton truck with 3" body lift with 38.5x16x16.5 tires on his! We used to go mudding all the time, and no problems! Now, I think with his 38's he ended up rubbing, and the sheetmetal just bent out of the way in front where the tires hit, but the truck drove just fine with them!
I've also seen trucks that looked to be similar year and model with 6" suspension lift, and barely clearing 35's.
Is 4" truly 4" I guess is what the real question is.. With a body lift, you have no wiggle room, either the blocks are 3" or they aren't. With springs, well, it depends on if your originals were 2 leaf in front, or 3 leaf, etc, etc, how much more will the new springs flex and allow the tires to come up into the wheel well than the originals, etc, etc..
I have to admit, I like 4" lifts best as your geometry and drive shaft angles are close neough to stock you don't have to worry about undue wear on the u joints and so forth.
Seen way too many a dude with 6-8" lifts snapping u joints as soon as they get into the mud, and I'd have to pull 'em out with my old beater truck with cheapo body lift on it.. Never liked how the rust ate away right at the body lift mounts, or the gap between the bumper etc though..