A truck filled with explosives blew up Thursday at a crowded bazaar in the Afghan capital of Kabul, killing four people and injuring 18, Radio Kabul reported. The blast occurred shortly before Soviet Foreign Minister Eduard A. Shevardnadze arrived in Kabul for what was described a "working visit." Afghanistan's Radio Kabul, monitored in Islamabad, blamed the explosion on guerrillas fighting the Soviet-backed government. The Soviet news agency Tass, in a report from Kabul, said the explosion occurred in the Shahrara area of the city.