In astrological tradition the transition of the vernal equinox into the zodiacal sign of Aquarius (taking place towards the end of the 10th Cent.) will coincide with a new advance in human development.
Jung argues in Aion that whereas the aeon of Pisces was ruled by Christ and Antichrist (par. 141) and the archetypal motif of the hostile brothers. Aquarius will constellate the union of opposites (par. 142).
The “4th month” puts the beginning of world history into the aeon of Taurus, i.e., 6-7000 years ago, when man first became conscious of historical time (each “Platonic month” lasts about 2,000 years). Cf. Baur, 29 Jan 34, n1. ~Footnote, ~Carl Jung, Letters Vol. 1, Pages 69-70.