The term Sizheng is given after traditional Chinese philosophy concerned with Taiji, Yinyang, and Bagua, the fundamental theory of Taijiquan. The Taiji represents the two opposing elements, namely Yin and Yang, which are interacting and fusing with each other, producing two normal elements called Liangyi (兩儀). Consequently, Liangyi produces four main elements (four pillars), namely the Sixiang (四象), which are named Lesser Yin (少陰, shaoyin) and Greater Yin (太陰, Taiyin), Lesser Yang (少陽, Shaoyang) and Greater Yang (太陽, Taiyang).
So on the eight elements (八卦, Bagua) are born from Sixiang, which represent the fundamental essence and phenomena of the universe including human society.
In the theory of Taijiquan, Sixiang (四象) is also called Sizheng (四正), and represents four main basic Jin (Peng, Lu, Ji, An) and four main Jinglou of human body, which are the main channels or meridians in human body based on Chinese medicine. In fact, the Sizheng represents more, such as four normal directions in space, and four main ways of human morality, etc. Therefore, the Sizheng Taijiquan was created based on these philosophies.
More information on the Sizheng Taijiquan dedicated page