On the lawn of the White House, President George Bush led the nation in a minute of silence that began at 8.46am (1246 GMT) - the time when the first of four airliners hijacked by al-Qaida members slammed into New York's World Trade Centre.
A grim echo of that day was provided by a suspected American member of al-Qaida who threatened attacks against Los Angeles and the Australian city of Melbourne in a videotape shown on Sunday by ABC television.
The threat was made by Adam Gadahn, a US national who converted to Islam and became a supporter of Osama bin Laden, ABC said.
At Ground Zero, the focus of the day's remembrance events, the moment's silence was followed by brothers and sisters of the victims reading out the names of the 2749 people who died in the attacks on the twin trade centre towers.
"Again, we are a city that meets in sadness," said New York Mayor Michael Bloomberg
"We come here to remember the names of those we lost four years ago. The greatest honour we can do them is to remember them, not just as they were in death, but as they were in life," said Bloomberg, who also paid tribute to those killed in the recent terror attacks in London.
Hundreds of relatives gathered at the site, many of them holding pictures of their loved ones with messages like: "Always missed, never forgotten," and "God bless my son."
"We come here to remember the names of those we lost four years ago. The greatest honour we can do them is to remember them, not just as they were in death, but as they were in life"
New York mayor
During the roll call of the dead, family members descended into the pit of Ground Zero to lay flowers in two small reflecting pools representing the footprints of the twin towers.
This year, the thoughts of many Americans were 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.
"To Americans suffering in the aftermath of Hurricane Katrina, our deepest sympathies go out to you, this day," Bloomberg said.
Bush, under fire for the sluggish federal response to the disaster, was scheduled to visit the hurricane-devastated region later in the day.
Both tragedies served to highlight the vulnerability of the world superpower, but where 11 September forged a sense of national unity out of trauma and anger, Katrina highlighted divisions of race and class and triggered accusations of government indifference.
Ceremonies were also held at the Pentagon for the 184 people who died in the attack there, and in the field in Pennsylvania where a fourth airliner carrying 44 people crashed after passengers staged a rebellion against the hijackers.
Memorials have aimed for a more
low-key, personal tone
In Washington, a Defence Department-sponsored Freedom Walk in Washington to honour US troops in Iraq was scheduled to begin at 10am (1400 GMT), in what critics called a piece of political propaganda by the Bush administration.
Among the senior officials attending the Ground Zero ceremony were New York Governor George Pataki and Secretary of State Condoleezza Rice, who read a work by 19th century poet British Christina Rossetti.
Since the emotional intensity of the first 11 September 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 offered 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 cult cartoon sitcom The Simpsons.
Broadway shows were open as usual and the fashion industry converged on Manhattan's Bryant Park for the third day of the annual Fashion Week.
Also on Sunday, several thousand people took part in a march organised by the Pentagon to mark the fourth 9/11 anniversary, but it was denounced by critics as a stunt to build support for the Iraq war.
The Freedom Walk sponsored by the Department of Defence began shortly after 10am (1400 GMT) at the Pentagon and wound its way to Washington's National Mall, where a concert was given by country music star Clint Black.
Leah Levy, 58, a federal employee, said she had turned out to show her support for Bush and his policies.
"I believe in this president who is trying to make this world a safer place. I support (the war in Iraq) 100 percent"
"I believe in this president who is trying to make this world a safer place," she said. "I support [the war in Iraq] 100%."
Pentagon officials took drastic measures to control participation in the march with the route fenced off and lined with police.
The estimated 10,000 people who took part all had to register online beforehand and city officials warned that anyone seeking to join without the proper credentials would be arrested.
But at least two protesters wearing T-shirts with messages opposing the war in Iraq managed to take part, although they were closely followed by US Secret Service agents.
Anti-war groups denounced the event as a way to boost the war effort in Iraq and link events there with the September 11 attacks.