"After completion of the investigation, the charges against four of the defendants have been changed to corruption on earth," said Abbas Jafari Dolatabadi, cited by Mizan Online, a news outlet run by the judiciary.
"Corruption on earth" is one of the most serious charges in Iran and can be punishable by death.
The defendants were initially arrested on suspicion of espionage, but Mizan Online did not specify the charges that had preceded Wednesday's alteration.
Indictments for eight environmental activists have been issued in total and "are ready to be sent to court," the agency quoted the prosecutor as saying.
"The defendants had been notified of the charges," he added.
The prosecutor did not specify the charges against the other four activists, nor did he say when the trial would be held.
The authorities have not released the names of the defendants, who were arrested in January.
Iran has arrested at least a dozen environmental activists this year on charges of spying for foreign intelligence agencies.
One of them, Kavous Seyed Emami, a 63-year-old professor and renowned environmentalist, allegedly committed suicide in prison in February, a fortnight after his arrest.
Military authorities told the judiciary that the defendants had tried to "get close to military centres and get military information about these centres, under the cover of environmental activities," the prosecutor said.
In early September, the head of Iran's Environment Department, Isa Kalantari, called for the fate of the environmental activists to be explained.
"We are not asking for their release or for their execution, we just want their fate to be clarified. This is part of their citizenship rights," Kalantari said.
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