Lucas Rodrigues netted his third goal in as many games on the stroke of half-time to give the Hornets the lead.
In-form Horsham marksman Jack Mazzone then doubled Horsham’s lead on the hour.
Bognor’s afternoon went from bad to worse when Sam De St Croix saw red for aiming a kick at Rodrigues.
The capacity crowd at the Camping World Community Stadium were treated to another two goals to cap an excellent Hornets display.
Mazzone added his second of the afternoon on 71 minutes before substitute Daniel Ajakaiye added the icing on the cake at the death.
The win, coupled with Horsham’s 1-1 draw at Margate on Friday night, moves Dominic Di Paola’s side up to fourth in the Isthmian Premier.
The Hornets are a point adrift of place-above Folkestone Invicta and two points behind league leaders Potters Bar Town and second-placed Aveley after five games.
See some pictures by the club's photographer John Lines on this page and the ones linked.
Get the latest from the Camping World Community Stadium in the West Sussex County Times - out every Thursday.
1. Horsham 4, Bognor Regis Town 0 - the match in pictures
Match action from Horsham's West Sussex derby win over ten-man Bognor Regis Town on August bank holiday Monday Photo: John Lines
2. Horsham 4, Bognor Regis Town 0 - the match in pictures
Match action from Horsham's West Sussex derby win over ten-man Bognor Regis Town on August bank holiday Monday Photo: John Lines
3. Horsham 4, Bognor Regis Town 0 - the match in pictures
Match action from Horsham's West Sussex derby win over ten-man Bognor Regis Town on August bank holiday Monday Photo: John Lines
4. Horsham 4, Bognor Regis Town - the match in pictures
Match action from Horsham's West Sussex derby win over ten-man Bognor Regis Town on August bank holiday Monday Photo: John Lines