Reference: Quaternion
Easton
a band of four soldiers. Peter was committed by Herod to the custody of four quaternions, i.e., one quaternion for each watch of the night (Ac 12:4). Thus every precaution was taken against his escape from prison. Two of each quaternion were in turn stationed at the door (Ac 12:6), and to two the apostle was chained according to Roman custom.
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Fausets
A guard of four soldiers, two attached to the prisoner, two outside his cell door. Four quaternions took by turns the guard over Peter for the four night watches (Ac 12:4).
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Morish
A party of 'four soldiers.' Four such parties were told off to guard Peter when in prison, that they might relieve each other every three hours in the night. Ac 12:4.
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Smith
Quaternion,
a military term signifying a guard of four soldiers, two of whom were attached to the person of a prisoner, while the other two kept watch outside the door of his cell.