Why does a person's birthday become a "eight-character" in astrology?

The four pillars, also known as horoscope and birthday, refer to the time when a person was born, that is, year, month, day and hour. When people use heavenly stems and earthly branches to express year, month, day and hour respectively, such as Jiazi Year, Bingxu Month, Xin Chou Day, Renyin Time, etc. There are two words in each column and eight words in four columns, so the four-column prediction is also called "approving eight characters", "measuring eight characters" and "eight-character fortune telling". According to heavenly stems and earthly branches's relationship between Yin and Yang and the five elements, we can infer people's good or bad luck.