The London Nationals topped the LaSalle Vipers 6-2 in Game 4 on Tuesday.
James Turner led London with two goals, while Christian Polillo, Isaiah Payne, Max Vinogradov and George Diaco all scored once.
Jack Bowler and Zane Dalpe scored the two goals for LaSalle.
The Nationals now lead the best-of-seven series 3-1.
The Vipers will look to stave off elimination in Game 5 on Wednesday. The puck drops at 7 p.m. in London.
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The Leamington Flyers fell 5-2 to the Komoka Kings in Game 4.
Thomas Michaud and Johnny Ulicny each scored once for Leamington.
The Flyers are one loss away from elimination with the Kings leading the best-of-seven series 3-1.
The two teams face off in Game 5 on Thursday in Leamington at 7:10 p.m.