4 cannot be the measure of the third side. This is because of the Triangle Inequality Theorem, which states that the sum of two sides of a triangle must be greater than the third side (A+B>C, A+C>B, B+C>A) In this example, if side C were 4, side C (4) plus side A (8) would be 12. Since side B is 12, and 12 cannot be greater than 12, 4 would not work.
