The explosion completely destroyed a car, which was going to the South Waziristan tribal region from the northwestern town of Lakki Marwat.
“All four bodies are beyond recognition and it is difficult to ascertain their identity,” district police chief Ali Akbar told AFP.
The police officer added that it was still unclear whether explosives were being carried in the vehicle or if somebody planted a bomb underneath the car.
“However, we are investigating to get to the bottom of the incident,” Akbar said.
Another senior local police official, Shahid Hameed, also confirmed the incident and casualties.
South Waziristan is one of the seven semi-autonomous tribal regions where the Pakistani military has been battling Taliban and Al-Qaeda-linked militants.
Pakistan’s Islamist insurgency began after the US-led invasion of neighbouring Afghanistan in 2001 which led to a spillover of militants across the border and a surge in recruitment for Pakistani extremist groups.
The Pakistani army launched Operation Zar-e-Azb in 2014 in a bid to wipe out militant bases in the North Waziristan tribal area and bring an end to the near decade-long insurgency that has cost Pakistan thousands of lives.
Thursday’s incident comes days after a Taliban suicide bomber killed more than 70 people including many children in a park crowded with families in Lahore.