Boyd’s Bible Dictionary:
(file of four). A Roman guard of four soldiers two of whom watched prisoners within the door, and two watched the door outside (Acts 12:4-10).
Concise Bible Dictionary:
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 (Acts 12:4).
Strong’s Dictionary of Greek Words:
Transliteration:
tetradion
Meaning:
neuter of a presumed derivative of τέτρας (a tetrad; from 5064); a quaternion or squad (picket) of four Roman soldiers