The three men and a woman were being questioned about the 1972 no-warning car bombing in the village of Claudy.
Two men, aged 67 and 60, were arrested in the village of Dungiven not far from Claudy and another man aged 50 was arrested in Portglenone, Co. Antrim. The woman, aged 58, was arrested in Dungannon, Co. Tyrone.
Nine people, including a nine-year-old girl, were killed in three no-warning car bomb explosions in Claudy, but the bombers were never caught.
In a later police investigation, started after the 30th anniversary of the bombing, it was claimed a Catholic priest, the Rev. James Chesney, who died in 1980, had been involved.
Documents from the time showed that the then Northern Secretary, Willie Whitelaw, expressed anger to the Catholic primate of Ireland, Cardinal William Conway.
Chesney was moved from his parish across the border after the bombing when rumors spread that he had been involved in providing an alibi for one of the suspects.