In the Book of Changes, Zhen represents the eldest son, Kan represents the second son and Gen represents the third son. If there are four sons, what are their positions?

Earthquake represents the eldest son, Kan represents the second son, and Gen represents the third son. If there are four sons, the fourth son has the characteristics of both the eldest son and the youngest son, representing the position of the fourth son or the position of the root, but the position of the root is specific, specifically the position of the yin in the root (the root has two branches of yin and ugliness, and the position of the yin represents the fourth son).