The four men were killed in an attack this week on the U.S. Consulate in Benghazi, Libya.
President Barack Obama and Secretary Hillary Clinton gave remarks at the transfer of remains in an airport hangar, where four hearses with rear doors open waited for the caskets. Families of the victims and dignitaries were seated facing the hearses and a lectern.
"Four Americans, four patriots, they loved their country and chose to serve it and serve it well," the president said. "They had a mission and they believed in it, and the knew the danger and they accepted it. They didn't simply embrace the American ideal, they lived it, they embodied it: the courage, the hope and, yes, the idealism.
"We will bring to justice those who took them from us," Obama said.
Clinton said the four victims' lives -- as well as all people who work in the Foreign Service -- "are at the heart of what makes America great and good.
"America must keep leading the world. We owe it to the these four men to continue the long, hard work of diplomacy," Clinton said. "We will wipe our tears and stiffen our spines and face the future undaunted."
Defense Secretary Leon Panetta also was to be present, Pentagon spokesman George Little said.
Four people have been arrested in connection with the attack that left Stevens and the three other Americans dead, the top aide to the president of the Libyan parliament said Friday.
Those arrested were not directly tied to the attack, Monem Elyasser, the chief aide to Prime Minister Mustafa Abushagur, told CNN by telephone.
Elyasser did not release the identities of the four suspects in custody, nor did he detail the allegations against them.
The announcement came as the United States is struggling to determine whether a militant group planned the attack that killed the four Americans.
The head of Libya's ruling General National Congress, Mohammed Al-Megaryef, also confirmed the four arrests but declined to say to what group the suspects are linked.
However, the government now believes the suspects are part of one of the many armed extremist groups operating especially in the eastern part of the country and Benghazi itself, he told CNN.
Authorities also believe the attack was planned and deliberately carried out to inflict maximum damage on key Western interests, particularly the United States, he said.
The government believes the attack was intended to drive a wedge between Americans and Libyans.
Asked what the Libyan government was doing to ensure security, al-Megaryef answered, "We are doing our best to avoid further attacks." But he acknowledged that authorities had little capacity to defend against the powerful extremist groups.
State Department Under Secretary Patrick Kennedy has said that the attack appeared to be planned because it was so extensive and because of the "proliferation" of small and medium weapons at the scene. He was briefing congressional staffers when he offered that theory.