Joigny, France ? Fabian Cancellara says he knows his days in the Tour de France’s yellow jersey are numbered as the race heads toward the Alps.

The Swiss time-trial ace kept the prized race leader’s shirt after Wednesday’s fourth stage, a mostly flat 119.9-mile ride from Villers-Cotterets to Joigny won by Thor Hushovd of Norway.

No rider has threatened Cancellara’s lead since he won Saturday’s prologue and extended his margin with a daring win in stage three Tuesday, but others are stronger climbers.

“For me, when I get into the mountains, it’s sure that it’s finished,” Cancellara said of his run as the leader.

Hushovd, who won the green jersey awarded to the Tour’s best sprinter in 2005, overcame stomach pain to earn his fifth Tour stage win with a final sprint, finishing ahead of South Africa’s Robert Hunter in second and Oscar Freire of Spain in third.

“Everything took place superbly well,” said Hushovd, who took first in 4 hours, 37 minutes, 47 seconds. “I’m too happy.

“I was sure my form wasn’t too bad.”

The Norwegian climbed from ninth to second place and gained bonus points for the victory that put him 29 seconds back of Cancellara. Andreas Kloeden of Germany, who was runner-up to Lance Armstrong in 2004, is third, 33 seconds behind.

Riders break out of the flats today with a 113.4-mile trek from the Burgundy town of Chablis to Autun featuring eight medium-grade climbs.

“Someone’s really going to have some guts to go for it tomorrow,” said Cancellara’s CSC teammate Christian Vandevelde of the U.S. “People are going to be biding their time thinking about (the Alps).”

In the Alpine stages of the three-week race, overall favorites could try to make their move and chisel out precious minutes on their rivals. Few can be ruled out yet: 160 riders are within two minutes of Cancellara.

Kazakhstan’s Alexandre Vinokourov is widely seen as the top contender, though Kloeden, U.S. rider Levi Leipheimer, Australia’s Cadel Evans, Russian Denis Menchov and Spaniards Alejandro Valverde and Oscar Pereiro are all strong possibilities.