What is a…
quaternion

This is a band of 4 soldiers.

The Apostle Peter was committed by Herod to the custody of 4 quaternions, i.e., one quaternion for each watch of the night (Acts 12:4).

Thus every precaution was taken against his escape from prison. Two of each quaternion were in turn stationed at the door (12:6), and to two the apostle was chained according to Roman custom.