fourth son
Used in family contexts to refer to the fourth-born male child. Often appears in formal or written descriptions of family composition.
He is the fourth son and has three older brothers.
The name of the fourth son was also recorded in the family tree.
Compound of 四 (four) and 男 (son, male). The reading よん is a common on'yomi for 四, and なん is the on'yomi for 男 in this context.