If \( \lim_{x \to 0} \dfrac{\cos(2x) + a \cos(4x) - b}{x^{4}} \) is finite, then \( (a + b) \) is equal to:
\( \tfrac{3}{4} \)
\( -1 \)
\( \tfrac{1}{2} \)
\( 0 \)