Question

someones mom has four sons..north,east and west...what is the name of fourth son

Answer 1

Question is --> Someone’s mother has four sons; three are named North, East and West. What's the name of the fourth son.

Answer :- name of fourth son is what . As you can see in question mention full stop after sentence " what's the name of fourth son." It means this is not an inerrogative sentence . It is just a statement saying about name of forth son .
Hence, name of fourth son is " what "

Answer 2

Solution :-

The question is 'Someone's mom has four sons, north, east and west. What's the name of 4th son'.

Name of the 4th son is 'What'. It is clearly mentioned in the question that What's the name of 4th son. So, there is no doubt about it.

In this question, it is not asked that what is the name of 4th son. It is stated that what's the name of 4th son.

So, name of 4th son is 'What'.