Challenger Librado Andrade collapses in agony after taking a left to the midriff from Quebec champ Lucian Bute on Saturday night. (Jacques Boissinot/Canadian Press)

Montreal boxer Lucian Bute answered any critics he might have had with an emphatic fourth-round knockout in a rematch over Librado Andrade to retain his super middleweight title on Saturday night.

Bute and Andrade engaged in one of the more exciting fights of the year, but the end was sudden at Pepsi Colisee in Quebec City.

Andrade had Bute pinned on the ropes early in the fourth, but the champion countered with a left hook up top that dropped the usually iron-chinned Mexican like a sack of potatoes.

Andrade winked to his corner before getting up, but after trying to get back in the fight, he was countered again. This time it was a left hook below the right ribcage, causing Andrade to wince in pain as referee Benji Esteves counted him out.

Bute (25-0, 19 knockouts) celebrated wildly in the middle of the ring, much to the delight of an estimated crowd of about 16,000.

Bute's first win over Andrade in October 2008 was tinged with controversy. He outboxed the rugged Andrade impressively for the first 11 rounds but found himself dead tired in the final round.

Bute fell over once from exhaustion, and then was dropped hard by Andrade. Referee Marlon Wright made the curious decision to choose that juncture to warn Andrade about being too far out of the neutral corner, buying Bute some crucial seconds to the final bell at Bell Centre in Montreal. Bute won the unanimous decision.

Andrade made good on his promise not to wait too long in the rematch, pressuring Bute from the first round on. There were plenty of exciting exchanges, and Andrade may have pulled out the second round.

Andrade dropped to 28-3 (21 KOs), as Bute made the fourth defence of the International Boxing Federation title he won on just over two years ago.

The bout was broadcast in the United States by HBO Boxing, the first time they have been to Canada for a fight in over 25 years.