Quaternion

a military term signifying a guard of four soldiers, two of whom were attached to the person of a prisoner, while the other two kept watch outside the door of his cell. (Acts 12:4)

A band of four soldiers. Peter was committed by Herod to the custody of four quaternions, i.e., one quaternion for each watch of the night (Acts 12:4). Thus every precaution was taken against his escape from prison. Two of each quaternion were in turn stationed at the door (12:6), and to two the apostle was chained according to Roman custom.