Watford put their poor recent run behind them as they hammered local rivals Luton Town at Vicarage Road.
Slaven Bilic's side had lost three of their previous four games to slump to 15th in the Championship table.
But they found their form when it mattered to equal their biggest local derby win, sparked by an early acrobatic close-range strike from Keinan Davis.
William Troost-Ekong's goal on the stroke of half-time for Watford was the killer blow for the visitors.
Joao Pedro and Ismaila Sarr then both got in on the act after the break as Watford posted their biggest home derby win, while matching the 4-0 scoreline they beat Luton by at Kenilworth Road in October 1997.