Noor Khan Nikzad, a spokesman for the police chief in Herat province, says the Tuesday afternoon explosion in Kushki Kuhna district also wounded six civilians.
He says the target of the attack was a local community leader named Abdul Hakim. Hakim and the four other victims were killed when the bomber detonated his explosives after he walked into a shop.
Western Afghanistan is one of the more peaceful areas of the country but it has been the scene of a rising number of attacks in recent years.
