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.
Complete rebuild axle/engine/everything swap in progress: Building stock width 44's with 302 from 01 Explorer. Motor has GT40p heads, Cobra Intake, and truck cam. Running A9L EEC IV stand alone engine from late 80's-90's Mustang GT. Big Bearing NP435 married to NP205.