The four were set to be arraigned in circuit court Friday on charges that include robbery, home invasion and official misconduct, the Chicago Tribune reports.
The newspaper says it has learned from multiple sources that the four will testify for the prosecution and have already provided grand jury testimony about their alleged involvement in a police Special Operations Section crime ring led by former officer Jerome Finnigan.
Finnigan's group is accused of making false arrests and committing robberies and home invasions under the guise of busting street gangs, drug dealers and collecting firearms.
A parallel investigation run by the U.S. attorney's office is expected to produce federal indictments.
Federal prosecutors and the FBI entered the case in 2007 to investigate whether police department supervisors attempted to cover up the scandal.