Rocky Road claims their budget boost can be used without new shocks by installing the BPE's. Keep in mind however that they only extend your shock length 1.25" while their lift is 2". If your shocks are nearly maxed out at full droop already, the extra 3/4" may cause them to limit your travel.
The other thing to think about is that the chances are pretty good that your ProComp 4" lift uses lift shackles to provide some of the lift height in the rear. The Rocky Road lift also uses lift shackles in conjunction with a small block. You obviously can't use both sets of shackles so you're either going to be missing out on the 1.5" the Rocky Roads provide, or whatever height your ProComps are currently providing. If your ProComp lift used the factory shackles, you can disregard this statement.
The rest of it is going to depend on how complete your current lift is. Going from 4" to 6" may also require you to address steering, bump stops, pinion angles, drop brackets, track bar length, etc...