Meanwhile, police thwarted a plot to bomb the capital, Kabul.
Some 876kg of explosives and 5000 fuses were discovered on Monday hidden in several sacks of onions in a vegetable market in the eastern city of Jalalabad, said Khalil Ullah, the police chief in Nangarhar province.
Two men were arrested and told the authorities they had planned to take the explosives to Kabul for an unspecified attack, he said. The two named some 30 accomplices in Kabul who were now being sought.
Ullah said the explosives had been smuggled across the border from Pakistan.
The news came three days after police prevented another attack on a major hydropower dam near Kabul.
Afghanistan's government has warned that Taliban and al-Qaida fighters have launched a campaign of violence to subvert crucial legislative elections in September.
More than 700 people have been killed since a major upsurge in fighting in March. The authorities have warned that the violence is likely to worsen ahead of the polls.
Old land mines
The four Taliban rebels were killed in Uruzgan province on Tuesday as they were burying the bomb next to a dirt track regularly patrolled by Afghan troops, provincial Governor Mohammed Khan said.
It was not clear why the bomb exploded while it was being prepared. Four assault rifles were found next to the bodies of the four insurgents, he said.
Taliban fighters have launched
several attacks to undermine polls
The blast under the army truck occurred on the same day fighters laid a land mine on a road in Paktika province, said army General Shahorgul.
These fighters are increasingly using roadside bombs to target US-led foreign and Afghan forces.
Often, fighters collect old land mines left over from a quarter-century of war and then rebury them on roads commonly used by the security forces, or use the explosives in them for improvised bombs.
Police station targeted
In a separate incident late on Tuesday, insurgents fired three rocket-propelled grenades at a police station in eastern Logar province's Charkh district but the grenades fell short of the building, said provincial police chief Khan Mohammed.
"Fortunately, the rockets didn't reach the building. There were no casualties and nothing was damaged," he said. "It was an attempt by the enemy to sabotage stability before the parliamentary elections," he said.
The attackers fled the scene, and no suspects have been arrested, he added.