The fourth son’s name is Wie. Context:

• The classic riddle wording is “A mother has four sons: North, West, and South. What is the name of the fourth son?” The answer is contained in the phrasing: the word “Wie” (German for “how”) is the actual fourth name when read as part of the sentence. In other words, the name is the word that completes the sentence itself. This riddle famously hinges on recognizing that the question is embedded in a statement.

If you’d like, I can explain the reasoning step by step or provide translations of the riddle in other languages to illustrate how the same trick works.