BAGHDAD - A roadside bomb killed four U.S. soldiers in Baghdad on Sunday, the military said, pushing the overall American death toll in the five-year war to at least 4,000. The grim milestone came on the same day that rockets and mortars pounded the U.S.-protected Green Zone, underscoring the fragile security situation and the resilience of both Sunni and Shiite extremist groups despite an overall lull in violence. A Multi-National Division — Baghdad soldier also was wounded in the roadside bombing, which struck the soldiers' patrol vehicle about 10 p.m. in southern Baghdad, according to a statement. Identities of those killed were withheld pending notification of relatives. The 4,000 figure is according to an Associated Press count that includes eight civilians who worked for the Department of Defense. Last year, the U.S. military deaths spiked along with the Pentagon's "surge" — the arrival of more than 30,000 extra troops trying to regain control of Baghdad and surrounding areas. The mission was generally considered a success, but the cost was evident as soldiers pushed into Sunni insurgent strongholds and challenged Shiite militias. Military deaths rose above 100 for three consecutive months for the first time during the war: April 2007, 104; May, 126 and June at 101. The death toll has seesawed since, with 2007 ending as the deadliest year for American troops at 901 deaths. That was 51 more deaths than 2004, the second deadliest year for U.S. soldiers. The milestones for each 1,000 deaths — while an arbitrary marker — serve to rivet attention on the war and have come during a range of pivotal moments. When the 1,000th American died in September 2004, the insurgency was gaining steam. The 2,000-death mark came in October 2005 as Iraq voted on a new constitution. The Pentagon announced its 3,000th loss on the last day of 2006 — a day after Saddam Hussein was hanged and closing a year marked by rampant sectarian violence. The deaths taken by U.S. soldiers in Iraq, however, are far less than in other modern American wars. In Vietnam, the U.S. lost on average about 4,850 soldiers a year from 1963-75. In the Korean war, from 1950-53, the U.S. lost about 12,300 soldiers a year. But a hallmark of the Iraq war is the high wounded-to-killed ratio, partly because of advances in battlefield medicine, enhanced protective gear worn by soldiers and reinforced armored vehicles. There have been about 15 soldiers wounded for every fatality in Iraq, compared with 2.6 per death in Vietnam and 2.8 in Korea. The deadliest month for American troops was November 2004, with 137 deaths. April 2004 was the next with 135 U.S. military deaths. May 2007 saw the third-highest toll. Last December was the lowest monthly death toll, when 23 soldiers were killed — one less than February 2004. Two factors have helped bring down violence in recent months: a self-imposed cease-fire by a main Shiite militia and a grass-roots Sunni revolt against extremists. But commanders often say there is no guarantee the trends will continue. Among the concerns: the strength of breakaway Shiite factions believed armed by Iran and whether Sunni fighters will remain U.S. allies or again turn their guns on American troops instead of al-Qaida. Civil strife also could flare again. Shiite militias are vying for control of Iraq's oil-rich south. In the north, the contest for the oil-rich city of Kirkuk could spark new bloodshed and should be the focus of intense "U.S. diplomatic and economic leverage to make sure it doesn't happen," said retired Gen. Barry McCaffrey at a speech in New York in March to mark the fifth anniversary of the U.S.-led invasion. There is also the question of Iraq's security forces and the slow pace of their training. American commanders would like to see the Iraqis take more of a front-line role in the fighting, but their ability to operate without American support could still be years away. "We are always quick to note that the progress is tenuous and that it is reversible," said the top U.S. commander in Iraq, Gen. David Petraeus, "and that there are innumerable challenges out there."