In the given triangle we have the following:
A. The sum of the inner angles of a triangle is always equal to 180, that is:

From this, we can solve for the sum of angles 1 and 2 and we get:

Now, we also know that angles 3 and 4 are supplementary angles and therefore their sum is 180 degrees, that is:

Solving for angle 4:

Equating both results we get:

Therefore, it is true that angle 4 is the sum of angles 1 and 2.
B. We have that the sum of angles 2 and 3 is:

If this were equal to angle 4 we would have:

B