Given the information that M is the brother of N, B is the brother of N, and M is the brother of D, we can deduce the following:

1. N is not the brother of B (Option 1), because there is no direct information connecting N and B as siblings.

2. N is the brother of M (Option 2), because both M and N are stated as brothers. This statement is not definitely true, as it is only an inference based on the information given.

3. N is not the brother of D (Option 3), because there is no direct information connecting N and D as siblings.

4. M is the brother of B (Option 4), which is the correct answer. Given that M and B are both stated as brothers of N, we can conclude that they are also brothers of each other.

Therefore, the only statement that is definitely true based on the given information is that M is the brother of B (Option 4).