U.S. death toll in Iraq reaches 4,000

• Story Highlights
• Four U.S. soldiers die when roadside bomb hits their vehicle
  
• As Iraq war enters sixth year, American death toll rises to 4,000
  
• At least 35 Iraqis killed Sunday
  
• Iraq national security adviser says war is "well worth fighting"
  

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

The four were killed when a homemade bomb hit their vehicle as they patrolled in a southern Baghdad neighborhood, the U.S. military headquarters in Iraq said. A fifth soldier was wounded.

The grim milestone comes less than a week after 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," said Rear Adm. Gregory Smith, the U.S. military's chief spokesman in Iraq.

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

Of the 4,000 U.S. military personnel killed in the war, 3,263 have died in attacks and fighting and 737 in nonhostile incidents, such as traffic accidents and suicides. Eight of those killed were civilians working for the Pentagon.

The numbers are based on Pentagon data counted by CNN.

Also Sunday, at least 35 Iraqis died as the result of suicide bombings, mortar fire and the work of gunmen in cars who opened fire on a crowded outdoor market. Nearly 100 were wounded in the violence.

Estimates of the Iraqi death toll since the war began range from about 80,000 to the hundreds of thousands. Another 2 million Iraqis have been forced to leave the country, and 2.5 million have been displaced from their homes within Iraq, according to the U.N. High Commissioner for Refugees.

Many of the Iraqis and U.S. troops killed over the years, like the four soldiers slain Sunday in Baghdad, have been targeted by improvised explosive devices -- the roadside bombs that have come to symbolize Iraq's tenacious insurgency. Watch how the bombs have become a deadly staple »

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."

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 the struggle has implications for "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. 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.

Senior U.S. military officials are preparing to recommend to President Bush a four- to six-week pause in additional troop withdrawals from Iraq after the last of the so-called surge brigades leaves in July, CNN learned last week from U.S. military officials familiar with the recommendations but not authorized to speak on the record.

The return of all five brigades added to the Iraq contingent last year could reduce troop levels by up to 30,000 but still leave about 130,000 or more troops in Iraq.

Al-Rubaie emphasized Sunday that any drawdown of U.S. troops "has to be based on the conditions on the ground."

But there has been too much "foot-dragging on key governance questions in Iraq," Sen. Ron Wyden, D-Oregon, said Sunday. "It seems to me you put off those troop withdrawals, you send exactly the wrong message to the Iraqis."

When conditions warrant the withdrawal of American troops, the Iraqis will say, "'Thank you very much, indeed,' " al-Rubaie said. "A big, big thank you for the United States of America for liberating Iraq, for helping us in sustaining the security gains in Iraq ... and we will give them a very, very good farewell party then."

Responding to recent remarks from U.S. presidential candidates that Iraqis are not taking responsibility for their future, al-Rubaie said his countrymen are making political and security gains.

"Literally by the day and by the week, we are gradually assuming more responsibility," he said, noting that Iraqis have taken over security in many provinces.

CNN's Mohammed Tawfeeq contributed to this report.