"We have the yellow jersey, we won a stage with the team ... and Richie can finish it off with a nice podium spot in Paris", Van Avermaet said.
"It was a very hard sprint, but we knew how to pull it off", Gaviria said.
Gaviria's compatriot Rigoberto Uran nearly lost time after being caught in a late crash around 5km from the finish, but he was paced back to the main group by his EF Education First-Drapac team to stay 10th, 35 seconds behind Van Avermaet.
Fernando Gaviria of Quick Step Floors has won his second stage of his debut Tour de France, crossing the line first ahead of Bora-Hansgrohe's Peter Sagan and Andre Greipel of Lotto-Soudal to win a hard-fought sprint at the end of today's Stage 4 in Sarzeau.
Mark Cavendish and his Dimension Data team-mates had been heavily involved in the lead-up to the sprint but the Manxman did not feature at the sharp end after appearing to be cut off by Dylan Groenewegen (LottoNL-Jumbo). And maybe we will see the next days on the climbs.More news: Donald Trump again says he’s a ‘very stable genius’ at North Atlantic Treaty Organisation summit
Riders were enjoying a calm sunny afternoon in the cycling hotbed of northwest Brittany until a pileup near the front of the peloton with just over 5K left that sent several riders to the tarmac.
Among the top contenders, last year's runner-up Rigoberto Uran of Colombia is 10th, 35 seconds behind.
There were no changes among the other title hopefuls.
Porte is 14th overall at 51 seconds.
"The peloton slowed to allow them back, which also saw the gap extend again - 2'40" with 40km to race - which in turn ensured the bonus-second sprint was neutralised with the time bonuses taken by the break.More news: 128 dead following blast at Pakistan election rally
Fans cheered and waved the black-and-white striped Breton flag on the roadside as the peloton rolled through the country villages.
Wednesday's fifth stage is a 204.5km lumpy ride from Lorient to Quimper that should favour one-day classics specialists such as Van Avermaet.
BMC Racing kept the Franco-Belgian breakaway under control, however, and with the pace down there was little to report through much of the stage.
"It won't be easy with the wind and there are many teams chasing the honour of wearing the jersey", Van Avermaet, who races with Monday's time trial winners BMC, told AFP just ahead of the Tuesday's departure.
"It depends on who wants to defend and who wants to go for it". There will be more time lost throughout the GC group before we hit the mountains.More news: Drake says Will Smith wins 'In My Feelings Challenge'
Froome is aiming to join Jacques Anquetil, Eddy Merckx, Bernard Hinault and Miguel Indurain as the only riders to win the Tour five times.
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