The militants planted roadside bombs on the Quetta-Chaman highway near Pishin and detonated two explosive devices when the police convoy was passing the area, officials said.
"The blast was so intense that four policemen were killed on the spot. The injured have been shifted to hospital for treatment," an official said.
Baloch nationalist groups and Islamist militants have intensified attacks in the troubled province since last year. They have mainly targeted government and security installations and security personnel.