Andy Nunn resigned as Farnham Town manager after this abject defeat on Tuesday night.
The former Aldershot Town player informed the committee of his decision and coach Dave Ward will take over as caretaker manager.
The evening had started with Alan Morgan giving Farnham an early lead, but it was soon cancelled out by Neville Gordon who went on to score all of Southall's four goals.
He completed his hat-trick inside 15 minutes and the visitors led 3-1 at the interval.
Gordon added his fourth in the second half and even though red cards for Marvin Elliott and Ben Owemy reduced Southall to nine men, Farnham could make no impression on the game.
Farnham now have to regroup and it's probably a blessing that a quirk of the fixtures has left them without a league game for the next two weeks.
Ash United's 100 per cent league record was ended by Sandhurst on Saturday, the local derby ending 2-2. Scorers: Sandhurst - Danny Wilde, Peter Mulvaney; Ash - Scott Joyce, Paul Bonner (pen).