The Gen IV shocks have a 5% stiffer spring rate (Front: 210 lb/in Vs. 200 lb/in on a Gen3). The damping on the Gen IV shocks was reportedly also re-tweaked slightly to match the new rates and re-tuning for PS2s. In the rear, the spring rates also went up 5%; from 500 lb/in (Gen 3) to 525 lb/in on the Gen IV.
If anyone is interested, I have a very nice pair of Gen IV front shocks with less than 2k miles on them for $425/pair plus shipping. I installed a set of lowering caps on them and was looking for a pair of like-new rears to match, when I decided to buy a new set of KW V2s instead (thanks Jon B.). So a set of four new machined aluminum lowering caps (good for 5/8"-3/4" lowering) is included with the deal. These shocks are pristine clean and are part #05181432AA with spring #05181439AA. PM me with your e-mail address and I'll send photos of both fronts. Original spring caps are also included. New these are around $800 each!