Hydro is already used for peaking power, at most we would retrofit things it increase peak power output where it's reasonable to do so.

Building 4 hours of storage is a rather extreme step that's also extremely unlikely. California already has a 40c/kwh cost differential between peak summer demand and the middle of winter, but nobody is building massive grid energy storage to smooth that out. Even over the course of a single day you could make 10c/kwh just from grid storage but nobody is building it. Rather we add peaking power plants because base load power + storage costs more than peaking power plants and that's likely to continue to be the case even in a world dominated by solar + wind power plants.