Victory at Malmo in the previous round had taken Hammarby top of the table, but only for 48 hours after Sirius ended a three-game winless run by beating strugglers Halmstad.
Opening the round, Hammarby again had the chance to go ahead on goal difference and put pressure on the Uppsala side, but despite dominating the first half an hour with seven shots to Elfsborg's zero, Bajen were unable to find the target, as Montader Madjed lacked composure in front of goal.
However, when they finally found their shooting boots six minutes before half-time, they made the most of it. Madjed's low cross from the right wasn't properly cleared, instead, Hampus Skoglund kept the pressure on with a pass to Frank Adjei, who dug out a cross for Paulos Abraham, who chested the ball down and volleyed home from close range.
In the second half, the visitors failed to mount a response, and two minutes before the hour mark, their task got a whole lot harder as Hammarby doubled their lead. The assist went to goalkeeper Warner Hahn, who bowled the ball out to the halfway line where Abraham took it on, making his way to the edge of the area from where he fired low to complete his brace.
With still over a quarter of the match still to play, the home side were 3-0 up and in the most comical circumstances - Elfsborg goalkeeper Theo Sander passed the ball out to Thomas Isherwood, who attempted to complete a one-two, but only succeeded in playing the ball past Sander and seeing it nestle in the bottom corner.
In the final 15 minutes, the visitors again coughed up possession inside their own half, allowing Adjei to square for Abraham, and though he was brought down inside the area, the ball ran on through for Madjed to steer it home and add Hammarby's fourth.
Elfsborg had a consolation to cheer with nine minutes to play, when Leo Ostman scored his fourth goal in his last five games, but the heavy defeat all but ends Elfsborg's hopes of qualifying for Europe - they sit six points behind Djurgarden with two matches to play and a vastly inferior goal difference, as the Boras side's seven-match unbeaten run was ended comprehensively.
Sirius will only need a point to return to the top of the table when they take on Kalmar on Sunday 8th.