In a now-familiar ritual, the names of the 2749 people who died when the twin towers of the World Trade Centre collapsed under the impact of two hijacked airliners will be read out at the main memorial event at Ground Zero.
This year, the thoughts of many Americans will be focused on other victims: the estimated one million left homeless and the thousands feared killed by the hurricane which devastated the US Gulf coast nearly two weeks ago.
Both tragedies served to highlight the vulnerability of the world superpower, but where September 11 forged a sense of national unity out of trauma and anger, Katrina highlighted divisions of race and class and triggered accusations of government indifference.
On Sunday, The New York Times reported that the crisis in New Orleans after Katrina deepened because of a virtual standoff between hesitant federal officials and besieged authorities in Louisiana.
Bush radio address
In his weekly radio address on Saturday, President George Bush, under fire for the sluggish official response to Katrina, invoked the national spirit of post-September 11 defiance and resilience in an effort to pull the country together in the hurricane's wake.
"Today, America is confronting another disaster that has caused destruction and loss of life. This time the devastation resulted not from the malice of evil men, but from the fury of water and wind," Bush said.
The September 11 memorial is
overshadowed by Katrina's crisis
"America will overcome this ordeal, and we will be stronger for it," he said.
The administration is also trying to rally support with a Defense Department-sponsored Freedom Walk in Washington to honor US troops in Iraq, in what critics called a blatant piece of political propaganda by the Bush administration.
The part of the Pentagon where a third jet crashed on September 11 was opened to the public for the first time on Saturday.
Even without Katrina, the new 9/11 anniversary falls at a time of growing uncertainty in the United States.
Public concern about the cost, human and economic, of US involvement in Iraq is increasing. Opinion polls show many Americans feel no safer from the threat of another terrorist attack.
In a minor departure from previous anniversaries, Sunday's memorial at Ground Zero would also reflect on those who have been killed in militant attacks outside the United States over the past four years.
The part of the Pentagon that was
damaged on 9/11 was reopened
US Secretary of State Condoleezza Rice will attend the ceremony, together with New York Governor George Pataki and New York Mayor Michael Bloomberg.
The event will begin with a minute of silence at 8:46am (1246 GMT), the exact time when the first hijacked airplane struck the North Tower.
The names of those who died in the twin towers will be read out by their brothers and sisters.
During last year's ceremony, the roll call of the dead was given by the victims' parents and grandparents, while in 2003 the task had been given to their children.
The reading of the names will be interrupted for three more moments of silence: at 9:03am (1303 GMT) to mark the South Tower attack; at 9:59am (1359 GMT) for the collapse of the South Tower; and at 10:29am (1429 GMT) for the destruction of the North Tower.
Since the emotional intensity of the first September 11 anniversary, subsequent annual memorials have aimed for a more low-key, personal tone and other events - once cancelled or postponed - have crept back onto the calendar.
Three years ago, television networks devoted their programming to a day of remembrance. This Sunday they are offering live coverage of the start of the American football season, including the opening game of the New York Giants, and the season premiere of The Simpsons.
Broadway shows will open as usual and the fashion industry will converge on Manhattan's Bryant Park for the third day of the annual Fashion Week.
President Bush on Friday honours
public safety officers who died on 9/11
The Ground Zero ceremony has been tainted by controversy over the plans for a memorial museum at the site.
More than 500 relatives of the September 11 victims rallied there on Saturday, protesting against proposals for an International Freedom Center that will show exhibits on freedom struggles around the world.
The demonstrating victims' families, who held up photos of their loved ones and banners that read "Preserve Sacred Ground," say the center will shift the focus away from those who died in the twin towers.
Among two countries to honour the victims on the anniversary were neighbours Cuba and Canada.
While Cuba remembered "the terrible death of thousands of innocent people in the attack on the twin towers", Canada offered its prayers for "our closest friends and allies the United States".