"On Friday, a vehicle touched off an Improvised Explosive Device (IED) in Khak-e-Jabar district and the explosion killed four civilians while injuring seven others aboard the vehicle," Xinhua news agency reported.
A woman was among the killed while the injured included two children, the report said.
No group has claimed responsibility for the incident.
The Taliban militant group has largely been using home-made IEDs to launch suicide attacks and roadside bombings against security forces but the lethal weapons also inflict casualties on civilians.
Nearly 1,600 civilians were killed and more than 3,300 others were injured in attacks in the first half of the year, according to a UN mission report.