Jamieson, Fausset and Brown on Zechariah 6:7

7. bay--rather, the "fleet" (or "strong"). As the "red" are not otherwise mentioned, the epithet "fleet" (as the Hebrew for "bay" ought to be translated) in Zec 6:3 seems to apply to all four, and here especially to the "red." Their office is to complete hereafter the work already in part executed by the previous three who have stilled Babylon, Medo-Persia, and Græco-Macedonia; namely, to punish finally the last great foe of Israel, the final form assumed by the fourth world kingdom, Rome, which is to continue down to the second advent of Christ. Hence they "walk to and fro through the earth," counterworking Satan's "going to and fro in the earth" (Job 1:7; 2Th 2:8, 9; 1Ti 4:1), in connection with the last awful development of the fourth world kingdom. Their "fleetness" is needed to counteract his restless activity; their red color implies the final great carnage (Eze 39:1-29; Re 19:17, 18, 21).