Iran's conservatives are fighting calls for reform
In a statement carried by the Iranian news agency IRNA, the Islamic Revolutionary Court said the four have been freed on bail of 2bn rials ($250,000) each.
They were among around 60 members of the Iran Freedom Movement (IFM) and another liberal group - the Religious Nationalist Alliance - who were detained last March in a wave of arrests ordered by the conservative judiciary.
All the accused IFM members have now been released on bail although a few people close to the organisation remain in prison.
The four released in the last two days are former Interior Minister Mohammed Tavassoli, Hashem Sabaghian, Khosrau Mansorian and Abolfazl Bazargan, the son of the movement's founder
The IRNA statement said a fifth member of the group, Mahmoud Naimpur, had been released earlier.
Correspondents say another leading member of the group, Ezatollah Sahabi, was also freed on bail last Saturday.
Mass detentions
The Iran Freedom Movement advocates a democratic government based on Islamic ideology, but not run by clerics.
It was banned but tolerated until March 2001, when it was outlawed by the conservative courts amid sweeping arrests.
These came just ahead of the re-election of pro-reform President Mohammad Khatami, who was backed by the IFM.
Thirty-one of the 60 detainees went on trial late last year, accused of plotting to overthrow Iran's Islamic regime.
The detentions have provoked condemnation from international human rights groups as well as reformists within Iran, including President Khatami.