It is not possible for such a triangle to be formed because it violates the <em>Triangle Inequality Theorem </em>which states that the sum of two sides of a triangle must be more than the third side.
8 + 4 = 12 which is larger than the other side of 4cm so this is fine
4 + 4 = 8 is equal to the other side of 8cm so this is not fine.
So long as the summed length of any 2 of the sides is not more than the third side, a triangle cannot be formed.
