Middle East concern has said that Amin Khaki, Hossein Barounzadeh, Mohammad Bahrami and Rahman Bahmani are due to appear in Ahvaz Revolutionary Court on 1st February for a re-trial, which is the result of an appeal.
The defendants are affiliated with the self-styled "Church of Iran" network.
The charity says that those arrested were taken away, blindfolded and interrogated for several hours by armed intelligence and security agents.
The four detainees are among eight people arrested at a picnic in Spring 2014, who were imprisoned on charges of "action against national security".
Miles Windsor, from Middle East Concern, told Premier: "Initially it was charges of action against national security, but the latest verdict that has come up states - and I quote - they 'held illegal meetings, named as house churches, on rotation in several cities, attempting to spread Christianity and convert Muslims."
The charge is "that they held illegal meetings named as house churches in rotation in several cities, like Ahvaz and Susa, attempting to spread Christianity and convert Muslims."
Each of them was sentenced to one year in prison, including the number of days they were held in custody, for spreading Christianity in Iran.
Mr Windsor also said some leaders are worried about the spread of Christianity: "Some elements are genuinely fearful to an extent that within Iran Christians are seeking to bring down the government.
"They don't understand what Christianity is all about and they think that there is some sense that Christians and these house group movements are in some ways subversive to the government."
Listen to Premier's Antony Bushfield speak to Miles Windsor here:
