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 (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.
Dicionário Bíblico Ilustrado de Easton (Easton's Bible Dictionary)
Dicionário Bíblico (Easton)