Hastings' Dictionary of the Bible (1898)
QUATER'NION. When Peter is said to have been delivered to four quaternions of soldiers, and to have passed through a first and second watch, Acts 12:4, 1 Kgs 16:10. it is to be understood that he was guarded by four men at a time - viz., two in the prison with him, and two before the doors - and that they were relieved every three hours, or at each successive Bound between two Soldiers. (From an old Roman Drawing.) watch of the night, by four
others, making in all sixteen men.