The Taliban last month abducted two French aid workers and three Afghans working for Terre d'Enfance, an agency helping children in southwestern Afghanistan.
One of the French workers, a woman named Celine, was released last month in what the Taliban said was a humanitarian gesture.
The militants have demanded the withdrawal of French forces from Afghanistan and the release of jailed Taliban by the Afghan government, and set a deadline of Saturday for a deal.
"We beg you to let Eric, Hashim, Rasul and Azrat live and to give them back their liberty," the aid agency said in an e-mail received in Kabul. The family names of the hostages have not been released.
"The message that you gave to Celine has been passed on to all French leaders and French people. It will be understood all the better if you decide to be merciful."
A Taliban spokesman, Qari Mohammad Yousuf, told Reuters by satellite phone on Friday that neither the Afghan not the French governments had contacted them to negotiate the release of the four remaining hostages.
"If our demands are not met then our shura (council) will decide their fate according to Taliban's policy," he said.
President Hamid Karzai said this week efforts were under way to secure the release of the French man, after calls from French President Jacques Chirac for him to intervene.
The five were kidnapped in the southeastern province of Nimroz, sandwiched between Iran and Afghanistan's opium centre of Helmand.
Italian journalist Daniele Mastrogiacomo, who was kidnapped last month, was freed after two weeks when Kabul released five senior Taliban prisoners.
But his Afghan driver was beheaded in front of him before his release and his translator was later beheaded when the Afghan government refused to free more Taliban.
The Mastrogiacomo deal drew criticism in Afghanistan and Italy for encouraging the Taliban to take more hostages.
The Taliban are also holding five Afghan health workers and have threatened to kill one soon unless the government starts peace talks.
- Reuters - Thomson Reuters Foundation
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