I do not really like this Wikipedia explanation, especially as Ba Zi is equally Astronomy, as Astrology; or actually more based upon Astronomy, with Astrology as an extrapolation. One must understand the movement of the earth, sun, moon, planets etc. before assigning significance or characteristics to them. But it is a simple beginning; please follow some of the links on the page. https://en.wikipedia.org/wiki/Four_Pillars_of_Destiny .