1. The question asks about the meaning of set B from problem 4. 2. Since the original problem 4 is not provided here, I cannot directly define set B. 3. Generally, in math, a set like B is a collection of elements defined by a property or rule. 4. To understand set B, you need to refer to problem 4 where it is defined or described. 5. If you provide the details of problem 4, I can explain what set B means in that context.