The 25-year-old was last seen alive on February 27 2005 after attending a party at Ballyhalbert Caravan Park.
A fourth arrest has been made by detectives investigating the disappearance of Co Down woman Lisa Dorrian.
A 48-year-old man was arrested in the Craigavon area in Co Armagh on Thursday.
He was questioned on suspicion of murder, assisting offenders and preventing a lawful and decent burial, and was released unconditionally later in the day.
Ms Dorrian, 25, from Bangor, was last seen alive on February 27 2005 after attending a party at Ballyhalbert Caravan Park.
Despite extensive searches in multiple locations and campaigning by her family, Ms Dorrian’s body has never been found.
Police believe she was murdered on the night of February 27 or in the early hours of the following morning.
On Wednesday, a 40-year-old man was arrested in Millisle, Co Down, on suspicion of assisting offenders and withholding information.
He was released on bail pending further inquiries several hours later.
Last week a 40-year-old woman was arrested in Bangor and a 42-year-old man was arrested in Scotland.
They were held on suspicion of murder, assisting offenders, withholding information and preventing a lawful and decent burial, before being released on bail pending further inquiries.
