Acts 12:4. When he had apprehended him, he put him in prison, and delivered him to four quaternions of soldiers to keep him.
The usual number of a Roman military night-watch was four, and the watch was changed every three hours. Thus during the twelve hours of night there would be four of these watches or “quaternions.” Of these two were in the prison (see verse 6) and two were sentinels before the door. See verse 10 and the latter part of verse 6.