It is simpler to purchase a separate propane carburetor, so that you can switch back and forth between fuels as needs dictate.
My newest chart shows a 3600RPM 4-5Kw Genset at full power should run around 3.5-4 hours on 20 lbs. That's only 4.8 gallons of fuel, you understand, and a 20 lb. cylinder never really contains 20 lbs of fuel.
Reduce the load on the Genset, and extend the fuel run time.
4Kw is a LOT of electricity.