The incident on Saturday tops two days of violence in Afghanistan in which three others died, including a US soldier, and 15 were injured in several attacks in the troubled southeast.
The latest victims, who worked for a non-governmental demining agency, were shot dead in western Farah province when their two-vehicle convoy was ambushed.
The governor of Farah, Haji Abd al-Hai Namati, said three deminers were killed at the scene and a fourth died later from his wounds after talking to police.
The deminers, who worked for the Organisation for Mine Clearance and Awareness in Afghanistan (OMAR), were on their way from Farah to neighbouring Herat province when they were attacked in Bala Buluk district, Namati said.
Namati said the armed fighters were driving a stationwagon and chased the two OMAR vehicles before spraying them with bullets.
"This is an unfortunate incident," Namati said. "This is not the
work of thieves or robbers. This is al-Qaida and Taliban behind this incident."
OMAR were not immediately available for comment on Saturday.
Targeting aid workers
Saturday's attack is the worst incident targeting aid-workers
since four Afghans working for the Danish Committee for Aid to Afghan Refugees (DACAAR) were killed in execution-style shootings in September in southeastern Ghazni province.
Farah is usually a quiet province with most of Afghanistan's
insurgency and violence concentrated in the troubled south and southeast, along the porous border area with Pakistan where remnants of the ousted Islamic hardline Taliban regime and their al-Qaida allies are believed to be regrouping.