Acts 12:3

3, 4. When a man engages in a wicked enterprise, his conscience makes him timid while left to himself; but the applause of the multitude enables him to drown the voice of conscience, and rush on madly to the end. Agrippa may have hesitated when he found his hands stained with the blood of an apostle; but when the people applauded, he hesitated no longer. (3) "And seeing that it was pleasing to the Jews, he proceeded to seize Peter also. But it was in the days of unleavened bread. (4) And having apprehended him, he put him in prison, delivering him to four quaternions of soldiers to guard him, intending, after the Passover, to bring him out to the people." A public execution during the feast of unleavened bread would have been exceedingly incongruous with the religious solemnities of the occasion: hence this delay.

The four quaternions of soldiers who guarded Peter consisted of sixteen men, each quaternion consisting of four. It was enough to keep four men on guard during each of the four watches of the night. They, together with the strength of the prison doors, were deemed sufficient for the utmost security.