QUATERNION (kwa-tĕr'nĭ-ŭn, Gr. tetradion). A detachment of four men (Acts.12.4). The “four quaternions” (KJV; niv “four squads of four soldiers each”), to whom the prisoner was committed, were the four patrols who took the four watches of the night. Two, no doubt, watched inside and two outside the guardhouse.
. A guard of four soldiers. RSV Squad
International Standard Bible Encyclopedia (1915)
The name given to a company of four soldiers of Herod’s army (Ac 12:4). To four such companies Peter had been handed over, who would take their turn of acting as guard over the prisoner, each of the four watches of the night according to Roman reckoning, which Herod Agrippa I would follow. In the castle of Antonia Peter was thus closely secured, in order that Herod, who had already killed James, the brother of John, with the sword (Ac 12:2), might, after the solemnities of the Passover, make sure of his death likewise. On the night before his intended execution he was sleeping in his cell between two soldiers, "bound with two chains," his left hand chained to one and his right to the other. The other two soldiers of the quaternion mounted guard before the door, and are spoken of as "the first and the second guard" (Ac 12:10) whom Peter and his angel guide had to pass on the way to liberty. The Greek word thus rendered is not found in the Septuagint or anywhere else in the New Testament.