The Northrop Grumman 4-bladed rotor system is mounted on the turbine 333 airframe, which has the power to turn 4 blades. I doubt that you would see this on the piston models though, as only the turbine airframe would really benefit from 4 blades.
ATPMBA, the Enstron was designed that way on purpose. It has a very high inertia rotor system, to prevent rotor RPM decay in the first place. If however you allow that rotor system to actually decay to low RPM (not too easy to do), it would indeed take a little time to get the RPM's back up.