Speaking to ITV4 on Wednesday about the La Planche Des Belles Filles, Thomas said: "It's hard, I've done it before". This was the 2019 Tour's first foray into the mountains a stage that the defending champion, Welshman Geraint Thomas, had described as "the big day".
Bernal, 22, was named as co-leader by principal Dave Brailsford and it looked like he would overthrow Thomas after gaining five seconds on him in Monday's third stage.
Thomas is seventh overall, 45 seconds behind leader Julian Alaphilippe of France.
Thomas took full advantage of the terrain to produce brutal acceleration and take fourth place, 1:44 behind Teuns, who snatched his maiden stage win on a grand tour.
"But my form of the moment earned me a place in the starting lineup". "I thought I had got away with it at the time and I didn't have any trouble when I got back into the race, no skin off or anything like that".
Thomas's Ineos team mate Egan Bernal of Colombia appeared less fresh and lost nine seconds to the Briton, prolonging the status quo in the team's shared leadership.
The Trek-Segafredo rider, who already has two Giro stage wins to his name after one this year and another in 2016, quickly overcame the disappointment of losing to Belgian Dylan Teuns for the stage win after crossing the line 11 seconds later after a 160.5-km ride, most of it in the day's breakaway. "I felt pretty good".
Patrick Bevin of New Zealand has withdrawn from the Tour de France ahead of the sixth stage after breaking two ribs in a crash.
Three of the overall contenders lost significant time as 2014 champion Vincenzo Nibali, Jumbo Visma captain Steven Kruijswijk and, to a greater extent, Frenchman Romain Bardet suffered on the slopes.