Quadrilateral $ABCD$ is inscribed in a circle with $\angle BAC=70^{\circ}, \angle ADB=40^{\circ}, AD=4,$ and $BC=6$. What is $AC$? $\textbf{(A)}\; 3+\sqrt{5} \qquad$ $\textbf{(B)}\; 6 \qquad$ $\textbf{(C)}\; \dfrac{9}{2}\sqrt{2} \qquad$ $\textbf{(D)}\; 8-\sqrt{2} \qquad$ $\textbf{(E)}\; 7$