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:
Parts already gathered for assembly: 2001 Ford 302, 1990 Mustang GT 5.0 ECU and wiring, NP435, NP205, Pair of axles from 04' F350 SuperDuty (Sterling 10.5 Posi rear, Dana 60HP Front-Both Disk Brakes 3.73 Gears), 37" H1 Tires, 9" lift TJ Coils for 2 link conversion in front, Stage 2 Clutch Kit, 3 Row Aluminum Radiator, Chevy Astro Steering box (4" front Stretch), XJ Rear Leaf Springs (6" Rear Stretch), and lots more.