(τετράδιον, from τετράς, ‘the number four’; Vulg._ quaternio, whence the English word)

St. Peter, arrested by King Herod Agrippa, was handed over to four quaternions of soldiers (Acts 12:4), probably at the fortress Antonia. A quaternion was a guard consisting of four men, two of whom would be chained to the prisoner in the cell, while the other two kept watch outside (cf. Philo, in Flaccum, 13; Polyb. VI. xxxiii. 7). The second two were apparently the ‘first ward’ (φυλακή), which had to be passed before the iron gate was reached (Acts 12:10). Four quaternions were required, as the night was divided in Roman fashion into four watches of three hours each.

James Strahan.