Victorinus of Pettau — On the Creation of the World (tetras)

Now the reason of the truth is shown why the fourth day is named tetras; why we fast even to the ninth hour, or even to evening, or why a carrying-over even to the next day should be made. This world therefore consists of four elements—fire, water, heaven, earth. These four elements make a tetrad of seasons. The sun also and the moon through the year's span make four seasons—of spring, summer, autumn, winter; and these seasons make a tetrad.

And that I may narrate further from that matter: behold four living creatures before God's throne (Rev. 4:6), four Gospels, four rivers flowing in paradise (Gen. 2:10), four generations of peoples from Adam to Noah, from Noah to Abraham, from Abraham to Moses, from Moses to Christ the Lord, the Son of God; and four living creatures (Rev. 4:7)—of a human, a calf, a lion, an eagle; and four rivers (Gen. 2:11)—Pishon, Gihon, Tigris, Euphrates. The human Christ Jesus, author of those things we have recalled above, was seized by the impious in a tetrad. And so because of His captivity we keep the tetrad, so that for the majesty of His works and the healthful times of His humanity seasons may run fruitful and calm—therefore we either fast or make a carrying-over.