CAMBRAI, France — German rider Tony Martin won the fourth stage of the Tour de France to take the overall lead, while Chris Froome and his main rivals safely came through the ride over treacherous cobblestones Tuesday. Martin, one second behind Froome overall before the stage started, broke away from Froome’s yellow jersey group some two miles from the end. Martin, 30, a three-time world time trial champion who finished second behind Rohan Dennis in the opening stage’s time trial, finally got the yellow jersey he craves and fell into the arms of his teammates after the stage. Especially after the final cobblestone section, when he had a flat tire and swapped bikes with teammate Matteo Trentin. Froome overcame one nervous moment when he almost lost control of his bike, but having come through some stress-filled days of racing unscathed, his confidence is growing as he bids to win back the title he won in 2013.