WASHINGTON — Marking a grim milestone, a determined President Bush declared yesterday the lives of 4,000 American military men and women who have died in Iraq “were not lost in vain.” The White House signaled anew that additional troops won’t be pulled out soon.

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

That number pales compared with those of other lengthy American wars, but it is much higher than many Americans, including Mr. Bush, ever expected after the swift American invasion of Iraq five years ago.

Bush proclaimed the end of major combat operations in Iraq in May 2003. Almost all of the American deaths there have happened 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,'” Mr. 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,” Mr. Bush said.

The news of 4,000 dead in Iraq came the week after the war rolled into its sixth year, dominating most of Mr. Bush’s presidency.

Almost 30,000 American service members have been wounded in the war.

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

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

“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,” House Speaker Nancy Pelosi said.

She said the cost to the American reputation is immense, and the threat to the economy at home is unacceptable. The Senate majority leader, Harry Reid, said the death toll is a reminder that the nation must get out of an “endless civil war and make America more secure.”