noun
fourth son
Used in family contexts to refer to the fourth-born male child. Often appears in formal or written descriptions of family composition.
彼は四男で、三人の兄がいます。
He is the fourth son and has three older brothers.
家系図には四男の名前も記されていた。
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.