BAGHDAD, Iraq (CNN)  -- Four U.S. soldiers died in a roadside bombing in Iraq on Sunday, military officials reported, bringing the American toll in the 5-year-old war to the grim milestone of 4,000 deaths.



Troops emerge from a bunker after receiving the "all clear" Sunday following a truck bombing in Mosul.



The four were killed when their vehicle was hit by an improvised explosive device while patrolling a neighborhood in southern Baghdad, the U.S. military headquarters in Iraq reported Sunday night. A fifth soldier was wounded in the attack, which took place about 10 p.m. (3 p.m. ET).



The U.S. milestone comes just days after Americans marked the fifth anniversary of the start of the war.



"No casualty is more or less significant than another; each soldier, Marine, airman and sailor is equally precious and their loss equally tragic," Rear Admiral Gregory Smith, the U.S. military's chief spokesman in Iraq, said.



"Every single loss of a soldier, sailor, airman or Marine is keenly felt by military commanders, families and friends both in theatre and at home," Smith said.



Of the 4,000 U.S. military personnel killed in the war, 3,263 have been killed in attacks and fighting and 737 in non-hostile incidents, such as traffic accidents and suicides.



Eight of the 4,000 killed were civilians working for the Pentagon.



Many of those killed over the years, like the four soldiers slain on Sunday in Baghdad, have been targeted by improvised explosive devices -- the roadside bombs that have been described as the weapon of choice for insurgents and a weapon that has come to symbolize Iraq's tenacious insurgency.



The existence of the Pentagon's Joint Improvised Explosive Device Defeat Organization has been developed to counter the threat of IEDs in Iraq as well as Afghanistan. The group calls IEDs the "weapon of choice for adaptive and resilient networks of insurgents and terrorists."  Watch how IEDs have become deadly staple in Iraq war »



Meanwhile, estimates of the Iraqi death toll range from about 80,000 to the hundreds of thousands, with another 2 million forced to leave the country and 2.5 million people displaced within Iraq, according to the U.N. High Commissioner for Refugees.



President Bush ordered U.S. troops into Iraq on March 19, 2003, after months of warnings that then-Iraqi leader Saddam Hussein was concealing stockpiles of chemical and biological weapons and efforts to build a nuclear bomb.



U.N. weapons inspectors found no sign of banned weapons before the invasion, and the CIA later concluded that Iraq had dismantled its weapons programs in the 1990s.



Hussein's government fell in early April 2003, and Iraq's new government executed him in December 2006.



The news of the 4,000 mark came on the same day that Iraq's national security adviser urged Americans to be patient with the progress of the war, contending that it is "well worth fighting" because it has implications about "global terror."



"This is global terrorism hitting everywhere, and they have chosen Iraq to be a battlefield. And we have to take them on," Mowaffak al-Rubaie said Sunday on CNN's "Late Edition with Wolf Blitzer."



"If we don't prevail, if we don't succeed in this war, then we are doomed forever," he said. "I understand and sympathize with the mothers, with the widows, with the children who have lost their beloved ones in this country.



"But honestly, it is well worth fighting and well worth investing the money and the treasure and the sweat and the tears in Iraq."



Nearly 160,000 U.S. troops remain in Iraq, and the war has cost U.S. taxpayers about $600 billion, according to the House Budget Committee.



The conflict is now widely unpopular among Americans: A CNN-Opinion Research Corp. poll out Wednesday found only 32 percent of Americans support the conflict. And 61 percent said they want the next president to remove most U.S. troops within a few months of taking office.



In the weekly Democratic radio address Saturday, Sen. Bob Menendez of New Jersey said President Bush "took us to war on the wings
What is the American death toll? what was the result of sunday who hit by an IED what american death toll Where were the four soldiers from? What number of Iraqi's have been killed? what did the security advisor say What was the vehicle hit with? what is the cap of deaths what is approximately of iraqis casualties
4,000 Four U.S. soldiers died in a roadside bombing Four U.S. soldiers 4,000 deaths. U.S. about 80,000 to the hundreds of thousands, urged Americans to be patient with the progress of the war, contending that it is "well worth fighting" because it has implications about "global terror." an improvised explosive device 4,000 about 80,000 to the hundreds of thousands,
question: What is the American death toll?, answer: 4,000 | question: what was the result of sunday, answer: Four U.S. soldiers died in a roadside bombing | question: who hit by an IED, answer: Four U.S. soldiers | question: what american death toll, answer: 4,000 deaths. | question: Where were the four soldiers from?, answer: U.S. | question: What number of Iraqi's have been killed?, answer: about 80,000 to the hundreds of thousands, | question: what did the security advisor say, answer: urged Americans to be patient with the progress of the war, contending that it is "well worth fighting" because it has implications about "global terror." | question: What was the vehicle hit with?, answer: an improvised explosive device | question: what is the cap of deaths, answer: 4,000 | question: what is approximately of iraqis casualties, answer: about 80,000 to the hundreds of thousands,