The remote-controlled device in Helmand province was thought to have been planted by rebels allied to the ousted Taliban rulers, provincial government spokesman Mohammad Wali told AFP on Monday.
"Two of our police and two civilians travelling with them were killed," he said. "We believe it was the work of the Taliban."
Helmand, on the border with southwest Pakistan, is one of the areas hardest hit by a Taliban insurgency launched after the group was toppled in a US-led invasion at the end of 2001.
More than 1300 people, many of them fighters, have been killed in attacks since January.
In neighbouring Zabul, a bomb went off as a US military convoy passed through but there were no casualties, the US military said.
"There were no casualties, no damage to equipment," said Colonel Jerry O'Hara, spokesman for the 20,000-strong US-led military in the country.
US National Security Adviser Stephen Hadley defended the Afghan government's amnesty offer to leading members of the Taliban but said on Monday that those who were guilty of war crimes would be held accountable.
Hadley was speaking on a visit to Kabul a week after Afghanistan held parliamentary elections in which a number of senior members of the Taliban stood for office.
Stephen Hadley visited Kabul
after the Afghan elections
"The reconciliation process is one in which the Afghan government quite rightly is, and should, take the lead.
"But I am told that people who are guilty of crimes will not be eligible for reconciliation and will be held responsible for those crimes," Hadley said.
Mullah Qalamuddin, a former head of the Taliban government's notorious Ministry for the Prevention of Vice and the Promotion of Virtue, stood for election to parliament in the eastern province of Logar.
Former Taliban foreign minister Mullah Muttawakil stood in the southern province of Kandahar, while former Taliban ambassador to Pakistan Mullah Zaif was released from the US military's prison camp at Guantanamo Bay, Cuba, without charge after almost four years in custody shortly before the 18 September polls.
"They were on the terrorist list and that is why they were taken into custody," Hadley said.
Some former members of the government had been released because they were not found guilty of a crime, while others might have been released because of intelligence they provided to the US "war on terror", he added.
The US freed former Taliban
envoy Mullah Zaif before the polls
Last year the Afghan government offered an amnesty to all but about 100 of the Taliban, saying that if they laid down their weapons they could take part in Afghanistan's political life.
"There are more serious leaders who are outstanding and not in custody than the three you described, and I am sure that a number of those will not be part of the reconciliation process, but again that is a decision that the Afghan government has the lead on," Hadley said.
Hadley was on a two-day visit to Afghanistan where he met President Hamid Karzai, the defence and interior ministers and visited US troops in the southeastern town of Gardez.