The number of U.S. soldiers to die in Iraq has reached 4,000, the U.S. military said on Monday, just days after the fifth anniversary of a war that President George W. Bush says the United States is on track to win.

The U.S. military said in a statement four soldiers were killed late on Sunday when a roadside bomb, the biggest killer of American soldiers in Iraq, exploded near their vehicle in southern Baghdad. One soldier was wounded in the attack.