Mara's double helps Southampton book FA Cup date at Liverpool
Mara scored twice in the space of six minutes after the break and Che Adams, who set up both the opening goals, then confirmed promotion-chasing Southampton's progress with an effort of his own.
Promotion remains Southampton's priority but after extending their unbeaten run in all competitions to 24 they will relish a crack at Jurgen Klopp's Premier League leaders.
Southampton were joined by Coventry City who beat Sheffield Wednesday 4-1 to earn a tie with non-league side Maidstone United.
Elsewhere, Leeds United needed extra time to defeat Plymouth Argyle 4-1 at a rain-soaked Home Park to advance into the fifth round, setting up a tie with either Aston Villa or Chelsea, who play their replay on Wednesday night.
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