The blast near a major hospital in the predominantly Shi’ite Muslim city was caused by a motorbike rigged with explosives, the military said in a statement, citing preliminary information.
There was no immediate claim of responsibility.
“The blast carries fingerprints of Daesh (Islamic State),” Basra Governor Asaad al-Edani said.
Policemen were collecting body parts from a minibus that was badly damaged by the blast, reports said.
The street was covered with broken glass and blood.
Bomb attacks in southern Iraq, where the bulk of the country’s oil is produced, are relatively rare.
Police and hospital sources said that 20 people had been wounded, in addition to the four fatalities.
Helped by a U.
-led coalition and Iran-backed forces, Iraq declared victory over Islamic State in December 2017, after driving it from swathes of territory where it had declared a self-styled Islamic Caliphate.
But the group continues to carry out sporadic attacks, particularly in the north, where Islamic State briefly seized a village on Sunday before being driven out.
Motorcycle bomb kills four in Iraq, Islamic State gets blame