(Updates with weekend violence, edits)
* U.S. death toll reaches 4,000
* Impact on U.S. election unclear
By Ross Colvin
BAGHDAD, March 24 (Reuters) - 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.
The deaths came on a day when the U.S.-protected "Green Zone", the government and diplomatic compound in central Baghdad, was hit by repeated rocket and mortar fire, part of an upsurge in violence in the capital and elsewhere.
The violence, in which dozens were killed, underscored the fragility of Iraq’s security. There has been an increase in attacks since January, although U.S. military commanders say overall levels of violence are down 60 percent since last June.
What impact the 4,000 milestone will have on a war-weary American public and the U.S. presidential campaign will be hard to assess in the short term, but war critics are likely to seize on it to boost their case for U.S. troops to be withdrawn.
The U.S. military dismisses such tolls as arbitrary markers.
"It is artificial in the sense that somehow the 4,000th tragic loss somehow will be different from the first," U.S. military spokesman Rear Admiral Greg Smith told Reuters in an interview last week.
Anthony Cordesman, a respected Iraq analyst at the Center for Strategic and International Studies in Washington, said the 4,000 death could trigger another wave of polarised debate.
"Those who oppose the war will see it as further reason to end it. Those who support it, will point to military progress and say that future casualties will be much lower," he said.
Although Americans are more preoccupied with domestic economic troubles, the Iraq war is still an important issue in the presidential campaign, with Democratic hopefuls Hillary Clinton and Barack Obama calling for a timetable for withdrawal.
Bush said in a speech marking the 5th anniversary of the war on March 19 that the United States was on track for victory and said withdrawing troops, who now number about 160,000, would embolden al Qaeda and neighbouring Iran.
He said he had no regrets about the war, which has pushed his approval ratings near the lowest level of his presidency, but acknowledged the "high cost in lives and treasure".
Bush launched the war in March 2003 hoping for a quick victory with minimal casualties. The Iraqi army was quickly defeated, but within months insurgent attacks had bogged down U.S. forces who struggled to develop a strategy to defeat them.
The 1,000th U.S. soldier to die was in September 2004, 18 months after the invasion and in the midst of a presidential election that returned Bush to office for a second term.
The toll climbed to 2,000 in October 2005 as Sunni Arab insurgents battled to oust the Baghdad government, and 3,000 in December 2006, before Bush unveiled a plan to send 30,000 more troops to Iraq to quell violence that has killed tens of thousands of Iraqis and displaced millions more.
"I doubt the 4,000 milestone will have the impact that the 3,000 did. The conventional wisdom then was that things were going badly," said Stephen Biddle, a senior fellow for defence policy at the Council on Foreign Relations in Washington.
"Today, by contrast, the public’s general perception of Iraq is less negative, and coverage for the last six months has tended to focus on the reduction in violence and U.S. casualties. The war has also been much less visible," he said.
But the weekend barrages on the Green Zone, which houses the U.S. embassy, and the continued attacks on U.S. troops may indicate that Iraqi militants are trying to change to that.
"Al Qaeda and extreme elements of the (Mehdi Army) have every incentive to find ways to raise the U.S. casualties between now and November and will be seeking ways to use bombings to raise the rate and number," Cordesman said, speaking before the latest U.S. deaths were announced. (Writing by Ross Colvin, additional reporting by Randy Fabi; Editing by Stephen Weeks)