Meaning
"a group of four" (tetra--, "four"), occurs in Act 12:4. A "quaternion" was a set of four men occupied in the work of a guard, two soldiers being chained to the prisoner and two keeping watch; alternatively one of the four watched while the other three slept. The night was divided into four watches of three hours each; there would be one "quaternion" for each watch by day and by night. Cp. the "guard" in Mat 27:65, Mat 28:11.