Did you already try to make the Pro-Comp work better? Did you check the bolt torque and grease the bushings, etc.? Any of your local 4x4 shops will be able to get you just the springs, same with many of the vendors in magazines and maybe even vendors here. Do some research and read some customer reviews of different kits before you buy, so you don't end up with the same situation. Some of the kits may have been designed to work with the shocks in their "kit". It may be worth buying another complete kit and then selling yours. Boomerang shackles may help soften the ride a little too. I'm sure some people on here can give you opinions of what they like, I prefer Rubicon Express, but it may be tough to find some springs right now as they are going through a bankruptcy. They are supposed to be opening again in a month or two I believe, maybe partnered up with another company.
Edit: 4" springs are naturally going to be a little stiffer due to their arch, and they to take a little time break in and soften up. Maybe go jump it a couple times.