The bomb was triggered by remote control as a vehicle with several members of the anti-Taliban militia was passing through Mattani area outside Peshawar, the capital of Khyber-Pakhtunkhwa province.
Four members of the militia were killed instantly, police officials said. Three others were injured in the attack. The blast occurred near a private school.
Army soldiers and paramilitary forces cordoned off the area as ambulances ferried the wounded to nearby hospitals.
This was the second bomb attack in Mattani in five days.
A bomb planted in a passenger van on June 5 killed seven persons.
The Taliban have repeatedly attacked members of a pro-government militia in Mattani area, where residents have supported action against militants.
In March, at least 37 people were killed and more than 40 injured when a suicide bomber targeted the funeral of the wife of a pro-government tribal elder.