Local police official Umar Hayyat said it was unclear who killed the men as they drove through the town of Hangu, 45 miles (70 kilometres) southwest of North West Frontier Province’s capital Peshawar. He said police were investigating.
Ghani ur-Rahman, Hangu’s former mayor, said the four men were money changers, adding that the motive - including whether the attack was sectarian - was not immediately clear.
Hangu and other towns in northwest Pakistan bordering Afghanistan have seen many sectarian attacks recently. On Monday, a bomb exploded at a Shiite mosque in the town of Dera Ismail Khan, killing four worshippers.
Scores of people have reportedly died in months of intermittent fighting between Sunni and Shiite Muslims in Kurram, a mountain valley next to the frontier.
Minority Shiites and majority Sunnis generally live in peace in Pakistan, but extremists on both sides often target each other’s leaders and activists.