Suspension lift, with out question. You have several options 1. Design & build your own kit. By this i mean find individual components that are each the best quality/ price.
2. by a cheap 300 rancho kit on ebay w/ shocks and blocks that going to ride rouch and perform like crap.
3. By an $800 kit from sky jacker or some one with 4 spring and shocks and get a better kit.
Personaly if i was doing 4" i would choose option 1 and these are the parts i would use:
1. 4" shackle flip from www.offroaddesign.com
gives the rear 4" of lift, and uses stock springs, no blocks! $185
2. Tuff country front 4" springs from www.jackit.com
Best spring for the value $198.98
3. Rancho shocks, will give you a decent ride and a good price prolly 200 bucks for 4 from www.jacit.com
4. Pro comp brake lines increases line length to compensate for lift, increases braking force. 150 total for front / rear from www.jackit.com
5. Steering arm, about 60 bucks from www.jackit.com
This is a vital part for a lifted chevy to keep steering geometry correct.
The total of all of these things turn out to be just about $800 and will perform better than any "off the shelf kit" and you can still up grade it later with better shocks, new shock mounts, crossover steering, more lift, sway bar dissconnects, the list is almost endless.
Is an awsome website for product reviews. All of the products just listed were tested by these guys, you can read the reviews on all of those components. That should help!