Limits Continuity And Differentiability

Let $f(x)=\left\{\begin{array}{cc}\frac{1-\cos 4 x}{x^2} & , x<0 \\ a & , x=0 \\ \frac{\sqrt{2}}{\sqrt{16+\sqrt{x-4}}} & , x>0\end{array}\right.$
If $\mathrm{f}(x)$ is continuous at $x=0$, then the value of $a$ is