Sunday's attack occurred on the busy road in northwestern Baghdad, which leads to the capital's airport where US forces are based.
Police captain Saadun Aziz told AFP one of the dead men was an Iraqi travelling in one of the 4WDs.
A doctor at al-Sadr hospital, Mazhar Abd Allah, told Aljazeera a family of three had also been caught in the crossfire. An Iraqi man was killed while his six-months pregnant wife and two children were wounded, he said.
Two witnesses told Aljazeera that the five or six survivors of the attack – some armed - seized another car at gunpoint to escape.
The dead convoy passenger, whose face was covered in blood, was wearing a combat vest and a pistol was resting on the floor of the vehicle, witnesses said.
It was not immediately clear if the man was an employee of the US-led occupation authority or a private contractor, but 4WDs are the vehicles of choice for occupation authority employees.
Witnesses said the assailants had opened fire with assault rifles from an overpass directly above the road. Two of the vehicles appeared to have crashed.