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. ) watch of the night, by four others, making in all sixteen men.