Wednesday's attack in the northeastern province of Kunar came on the eve of the second anniversary of the fall of the Taliban regime and appeared to be the work of armed fighters, residents said on Thursday.
They said the intended target in the incident in Manogi district of Kunar was probably a vehicle from the US-led forces driving just in front of the car that was destroyed.
"The only things you can see of the car are small burnt pieces," said one witness. "In total, four have been killed and three others wounded."
The blast was just two kilometres from a base used by US-led military forces and not far from the border with Pakistan. US forces in Kunar come under regular, but mostly ineffective, attacks by armed fighters.