Three of the contractors were killed instantly and a fourth died after British troops took him to a military hospital after the bombing on Wednesday in Basra, said Peter Mitchell, a US embassy spokesman in Baghdad.
"All four individuals worked for a private security firm supporting the regional US embassy office in Basra," Mitchell said in a statement.
Initial reports had indicated that the target of the attack was a British diplomatic convoy, but officials in London said no British personnel were involved.
An AP Television News videotape showed an overturned white sport-utility vehicle in a ravine next to a busy highway. Six British Army Land Rovers, with Iraqi police cars and two civilian ambulances were parked nearby. British soldiers were seen loading a body from the SUV into a military ambulance.
An estimated 20,000 civilians are believed to be working for private defence contractors in Iraq. More than 200 have died there, including 13 employed by US company Blackwater Security Consulting.
Southern Iraq, where about 8500 British troops are deployed, has been mostly calm since US and British forces occupied Iraq more than two years ago. However, violence has increased there in the past two months.
Yawar (R) urged Sunnis to take
part in the coming elections
On 16 July, a roadside bomb in Amarah killed three British soldiers and wounded two others. Two weeks later, two Britons, who worked for the security firm Control Risks Group, were killed when a roadside bomb exploded alongside a British diplomatic convoy in Basra.
Two British soldiers died on Monday in a roadside bombing west of Basra, bringing to 95 the number of fatalities British forces have suffered since the war began on 19 March 2003.
Baha al-Araji, deputy head of the Constitution Committee, said Iraq's new charter would be sent to the government printing house Thursday. He said it stood unchanged from the version sent to parliament by the drafting committee on 28 August after several deadlines were missed.
Iraqi President Jalal Talabani, a Kurd, said the version to be printed contained one revision, a bow to an Arab League demand that the constitution acknowledge the country's role as a founder of the pan-Arab organisation.
The document previously said Iraq was an Islamic country but omitted references to its Arab identity and its history as a key player in the Arab world. Arabs constitute more than three-fourths of Iraq's people.
Iraqis will vote on the charter in a 15 October referendum, but it is opposed by Sunni Arabs and many non-Sunni groups.
Five million copies of the constitution are to be distributed around the country with monthly food rations.
Vice President Ghazi Al-Yawar, a Sunni, said the constitution did not meet the Sunni's demands "100%" but encouraged its adoption. The task now, he said, was for Sunnis to engage in upcoming parliamentary elections to boost their representation in the legislature for a future effort to amend the charter.
The leaders spoke on Wednesday at a memorial for victims of the 31 August bridge stampede in which more than 900 people died trying to reach the Imam Mussa al-Kadhim mosque in northern Baghdad, where the imam is buried.
Major General Hadi Hassan Omran, an Iraqi Defense Ministry director general, was shot to death on Wednesday as he drove through Baghdad's southern Dora neighborhood, said Dr Muhanad Jawad at Yarmouk hospital.
The doctor also said attackers killed Colonel Ammar Ismail Arkan, an Interior Ministry commando, and wounded four bodyguards in Baghdad's western Ghazaliyah district.
Elsewhere, a pipeline carrying oil from a field near Khanaqin on the Iranian border was bombed on Wednesday, interrupting a source of crude to Baghdad's Dora refinery, police said. A fire was burning, and the full extent of the damage was not immediately known.