This question has been used as a riddle in places but let us dissect it slowly to understand it. It has possibly more than one answer. We don’t know the structure of the sentence is of intend or it’s an overlook.
When you look at the first word “Someone’s” this word as used can mean that someone is a person. This is because the word has the apostrophe (‘) and (s) normally is used to show a possession of a noun. So ‘’someone’’ could be an individual and if it’s an individual the answer to the question could possibly be “Someone”.
Second possible answer would be “East”. We are assuming that the mother named her fourth son after the fourth compass direction just as the preceding three sons. The compass points normally are North, West, South and East.
The third possible answer would be “What”. Surprise! It’s like saying she has four sons North, South, West and then the fourth is called What. See the question does not end with a question mark. So “What” could be a possible name of an individual. It is like saying; John is the fourth son. That sentence is valid and if you replace John with “What” the sentence would still remain correct.