Lynette White's body was found with more than 50 stab wounds

The two women and two men had been arrested early on Thursday on suspicion of false imprisonment, conspiracy to pervert justice and misconduct.

Last week, five former officers, four men and one woman, were held and released over the inquiry.

Three men jailed for the murder in 1990 later had their convictions quashed.

In 2003, Jeffrey Gafoor was imprisoned for life after admitting he stabbed the 20-year-old Cardiff prostitute 50 times at her flat in the city's Butetown area on Valentine's Day.

All four officers were being held in Swansea for questioning in connection with offences of false imprisonment, conspiracy to pervert the course of justice and misconduct in public office.

South Wales Police Assistant Chief Constable Stephen Cahill said that it was "another step closer to establishing the truth behind what happened in the original investigation".

Convictions overturned

He added: "We have made a commitment to seeing this investigation through and getting answers to what went wrong.

"Lynette's family deserves that, the victims who were wrongly charged and, in three cases convicted, deserve that, the public of South Wales and the force all deserve that.

"There is still a great deal of work to be done, but I can reassure everyone that we remain steadfastly committed to seeing this search for the truth through to its ultimate conclusion."

Steven Miller, Yusef Abdullahi and Tony Paris - who became known as the Cardiff Three - were jailed for life in 1990 for Miss White's murder.