is this a 4" long shackle or a 4" shackle lift?
a 4" long shackle will yield about a .5" lift over stock, I believe the stock yj shackle is 3" long.
a 4" lift shackle would measure about 8 inches long and is not at all ideal, it will drastically change your spring angle and the shackle angle. also suspensions with very long shackles are prone to bending springs and as mentioned harsh rides.