"Bazi 八字" is the abbriviated term used to mean the more academic term "Si Zhu Tui Ming Shu 四柱推命術". The term "Four Pillars" is the translation of "Si Zhu 四柱" and is, in fact, more picturesque.
Each year, month, day and hour is represented by a binomial of Tian Gan 天干 and Di Zhi 地支 which is to be distinguished as their functions and implications are different. We can say that one is the more visible part while the other is the more important background. Each part can represent a person/people or a situation/event.
The natal chart tells us the spiritual structure of a person, his character and the likely limits of achievement. This is largely influenced by the 10 year Pillar and each 1 year Pillar that comes after the person's birth. This accounts for the new environment and people the person will meet.
It is quite obvious that the character of the person leads to the probable reaction to what he encounters. However, we must not forget that education and experience can change the character of a person. This is why there is a part of life that we can control. In Chinese, there is a saying "人定勝天" (Man can surely win over Heaven). This does not mean that Human force can beat Heavenly force. It tells us that we can change the outcome of a situation brought to us by Heaven through our determination and effort.
Heaven can decide what to give us but cannot determine how we react and thus the outcome.