Ghana midfielder Andre Ayew came off the bench to score for Lorient to inflict a second home defeat in four days on St Etienne in emphatic fashion to leapfrog their opponents in the French top flight.
Jeremy Morel opened the scoring for the visitors after 19 minutes but Bafetimbi Gomis levelled just after the half-hour mark.
Bertrand Robert took the game away from Les Verts with a second-half double, before Andre Ayew finished off the scoring two minutes from time.
St Etienne could have taken an early lead after eight minutes but Lorient goalkeeper Lionel Cappone cleared Dimitri Payet's effort off his line.
And the visitors took full advantage as Morel counter-attacked, striding forwards before sending a left-foot shot into Jodi Viviani's goal.
Les Verts were level after 32 minutes, Geoffrey Dernis picking out Gomis in the box.
The France striker's header hit the bar, but bounced down into the goal despite Cappone's desperate efforts.
And they could have gone ahead had Stathis Tavlaridis done better with his header eight minutes before the break.
Lorient took control in the second half, though, with Yazid Mansouri latching onto Viviani's punched clearance and sending the ball inches over the bar.
After 55 minutes they took the lead for the second time as Kevin Gameiro countered before finding Robert who volleyed home from close range.
He grabbed his second 14 minutes later in similar circumstances, Gameiro again the supplier.
And a demoralised St Etienne conceded a fourth two minutes from time when Viviani inadvertently sent the ball into the path of Ayew, who gratefully finished into the empty net.
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