Protocol 4 was used by the Torchwood Institute to secretly assassinate an employee.
During her sabotage of Torchwood in 2005, Rachel Allan falsely accused Yvonne Hartman of murder and quickly entered her shoes, becoming the new director of Torchwood One. With her newfound power, she manipulated the grieving partner of the murdered employee to attempt to murder Yvonne under "Protocol 4". The attempt was unsuccessful, however. (AUDIO: Through the Ruins)