The shackle is what lets the spring compress and depress by letting it get longer and shorter. If your shackles are rusty than your ride will suck no matter what springs you have. Lift springs are usually a rougher ride until they are broken in, but if you never grease your shackle or spring bolts that doeskin really matter.
As for a 4" lift kit, if you use everything supplied in the kit you will be fine. If you want to wheel hard you will need to do a little more than just the kit though. Like SYE, and better brake lines for sure.
I'M FLEXY AND I KNOW IT
95 YJ, 2.5 AX15 conversion, 4.0 TB, SOA +1.5", 33" DC Crusher Tires, 8.8 Rear w/welded tubes + LSD, Spartan Locker HP30 Front w/solid axle swap, Rocky Road OTT kit, Safari Snorkel, Warn VR8000 Winch, Smittybuilt front Bumper, JKS Disconnects, AA SYE, Teraflex Shock Bar in Rear, 1/2" Boomerang Shackles, RE Add a Leafs, Homebrew Traction Bar, Monstalined in&out, LED Tail lights, 1700 Watt Stereo