Symons' side leapfrogged their second-bottom opponents after just their second league win of the season.
Hugo Rodallega, Fernando Amorebieta, Lasse Vigen Christensen and Tim Hoogland were on the scoresheet at Craven Cottage.
It marked Fulham's third straight victory in all competitions and could lead to Symons being appointed on a full-time basis.
Ross McCormack thought he had given the hosts an early lead when he tapped in, but the flag correctly went up for offside.
But Rodallega did open the scoring in the ninth minute as Fulham dominated proceedings.
Fulham had to wait until first-half additional time to double their advantage as Amorebieta rose highest to head in a Bryan Ruiz cross.
Christensen netted his first Fulham goal in the 67th minute as Ruiz and Rodallega combined before the latter set up the Dane.
George Williams squared for Hoogland to complete the rout with 11 minutes remaining.
Elsewhere, Huddersfield Town made it back-to-back wins and climbed four points clear of the bottom three with a surprise 3-1 victory at Wolves.
Harry Bunn and Sean Scannell scored in the first half before Conor Coady's 30-yard stunner.
Bakary Sako pulled a goal back for Wolves, who sit seventh.
Leeds United are winless in two matches under new head coach Darko Milanic after a 0-0 draw against Reading at Elland Road.
