The blast badly damaged the militiaman's car in a remote village in the district of Lower Dir, where a major operation was launched which was designed to dislodge a Taliban insurgency in 2009.
Sponsored by the government, tribesmen across the northwest have set up militias in a bid to drive the Taliban out of their communities.
"The bomb targeted anti-Taliban militia member Aziz-ur-Rehman. His 12-year-old son, a son-in-law and a neighbour who were also travelling in his car were also killed," local police chief Salim Khan Marwat told AFP.
"It was a remote-controlled IED (improvised explosive device) which exploded when the vehicle passed. Two people passing near by the car were wounded," said Marwat.
Another police official said the victims were travelling from their home village of Ghundo to the Samar Bagh area, 60 kilometres (37 miles) northwest of Timergarah, the main town of Lower Dir.
The Lower Dir district, 200 kilometres from the capital Islamabad, borders Pakistan's semi-autonomous tribal belt on the Afghan border, which Washington calls Al-Qaeda's global headquarters.
Nearly 4,700 people have been killed across Pakistan in attacks blamed on Taliban and Al-Qaeda-linked networks based in the tribal belt since government troops stormed a radical mosque in Islamabad in 2007.
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