And the four angels were loosed, which were prepared for an hour, and a day, and a month, and a year, for to slay the third part of men. The alterations in the Revised Version make the meaning much plainer: which had been prepared for the bout, and day, and month, and year, that they should kill, etc.
That is to say those "which had" in God's foreknowledge "been prepared" in order to operate at the exact period required—the exact year, month, day, and even hour. Each knew his appointed time. Four is the number used to denote universality in things of this world (see on Revelation 4:6).
The number, therefore, seems to imply that the power of the angels is of universal extent. The third part are destroyed; that is, a great part, though not the larger (cf. Revelation 8:7, et seq.).