This term means the four images, or features of the four images, which are engendered through the division of the two modes in the process of the formation of the eight trigrams. As explained in The Book of Changes, “Changes involve taiji (太极 the supreme ultimate), which produces two modes. The two modes generate the four images, and the four images generate the eight trigrams.” The four images are distinct from one another while also mutually related. There was no agreement among ancient scholars with regard to what the four images represent. From the point of view of the coming into being of all things, the four images might stand for the four seasons: spring, summer, autumn, and winter; or four basic elements: metal, wood, water, and fire. Alternatively, as a term in divination, the four images could refer to the four stalks in each group when the divination stalks are divided in a fortune-telling exercise, or to four line images for divination: greater yin, greater yang, lesser yin, and lesser yang.
The total number of divination stalks is fifty; those that are used are forty-nine. These are divided into two groups representing the pair of modes. One stalk is taken from one group to represent the three images of heaven, earth, and man, and the rest are counted out in fours to represent the four seasons. (The Book of Changes)
The four images generated by the two modes refer to metal, wood, water, and fire. They come into existence thanks to heaven and earth, and therefore it is said that the two modes generate the four images. (Kong Yingda: Correct Meaning of The Book of Changes)