Will the charger of the HP EliteBook 850 G5 work on an HP ProBook 450 G4?

look up the power specs for both models at HP website. if same voltage, the charger for the higher rated [power draw] model should work for both as long as connectors are identical

Maybe.  If plug shape and dimensions are same, if power block voltage and amps are the same or close, could work.
Another AC adapter for the ProBook is not expensive.