A US official close to the court said the order was to hold the witnesses to allow an investigation of their allegations of impropriety against the prosecution.
"Four defence witnesses have been ordered held," the official said on Wednesday.
The official said the inquiry would focus on witness allegations that Jaafar al-Musawi, the chief prosecutor, had attended an event celebrating a 1982 assassination attempt on Saddam.
Saddam and seven co-defendants are accused of crimes against humanity for bloody reprisals after the attempt on his life which led to the killings of 148 Shia men and teenagers from the town of Dujail.
Officials at the US-backed Iraqi court were not immediately available for comment.