Two persons died on the spot while another two succumbed to their wounds on way to hospital. The bomb went off at Chakkar Road, nearby a cobbler shop in the town of Sibi 130 kilometres (81 miles) southeast of the provincial capital Quetta.
Meanwhile, 13 others also sustained injuries who were moved to a hospital.
Senior police official Ghulam Ali Baloch, who gave the casualty toll, said two of the injured are in critical condition.
“The bomb was planted in a motorbike. We are investigating the possible targets but we have not found any specific target yet,” Baloch said.
Senior administration official Akbar Hussain Durrani confirmed the incident.