A roadside bomb in Baghdad killed four U.S. soldiers Sunday night, pushing the death toll to 4,000.

That number pales in comparison with the number of deaths in other lengthy U.S. wars, but it is much higher than many Americans, including Bush, ever expected after the U.S. invasion of Iraq five years ago.

Bush proclaimed the end of major combat operations in Iraq in May 2003. Almost all the U.S. deaths there have occurred since then.

"One day people will look back at this moment in history and say, 'Thank God there were courageous people willing to serve, because they laid the foundations for peace for generations to come,' " Bush said after a State Department briefing about long-term diplomacy efforts.

"I have vowed in the past, and I will vow so long as I'm president, to make sure that those lives were not lost in vain - that, in fact, there is an outcome that will merit the sacrifice," Bush said.

Also yesterday, the FBI said authorities had recovered the remains of two U.S. contractors who were kidnapped in Iraq more than a year ago.

Ronald Withrow of Roaring Springs, Texas, and John Roy Young of Kansas City, Mo., were among six Western contractors abducted in separate incidents - Withrow in January 2007 and Young in November 2006.

Their disappearances received new attention earlier this month when the severed fingers of several men were sent to the U.S. military in Iraq. Withrow, who worked for JPI Worldwide, was among those whose severed fingers were sent; Young, who worked for Crescent Security Group, was not.

The news of 4,000 U.S. troops dead in Iraq came a week after the war entered its sixth year. Almost 30,000 U.S. service members have been wounded in Iraq.

Tens of thousands of Iraqi civilians have been killed.

Early in April, Bush is expected to announce the next steps in the war, and he is likely to embrace a pause in troop withdrawals beyond those due to end in July.

Democrats in Congress and on the presidential campaign trail continue to push for a faster end to war. But Bush still has the upper hand for 10 months.

House Speaker Nancy Pelosi (D., Calif.) said: "Americans are asking how much longer must our troops continue to sacrifice for the sake of an Iraqi government that is unwilling or unable to secure its own future." The cost to the U.S. reputation is immense, she said, and the threat to the economy at home is unacceptable.