The attack happened in Kandahar city, said Col. Steve Bowes, a Canadian commander with the coalition.
The four were travelling in a Landcruiser four-wheel-drive when the suicide bomber in a Toyota Corolla attacked them, Bowes said.
Two of the Britons were in a serious condition, while the other two were lightly wounded, Kandahar Gov. Asadullah Khalid said.
He said the four were civilians were working on a customs project in Kandahar, a former stronghold of the Taliban.
The suicide attacker died in the assault, the governor said.
The Britons' vehicle was burned in the attack. Canadian troops based in the city cordoned off the area and took the four to hospital.
The bombing was the third suicide attack in Afghanistan in two weeks.
The spate of attacks comes amid a major upsurge in violence across much of the country since March that has left more than 1,300 people dead.Reuse content