$$\cos\ 4\theta+\sin\ 4\theta=\cos\ 4\theta+3\div4$$
$\theta =\frac{2\pi n_{1}+\arcsin(\frac{3}{4})}{4}\text{, }n_{1}\in \mathrm{Z}$
$\theta =\frac{2\pi n_{2}+\pi -\arcsin(\frac{3}{4})}{4}\text{, }n_{2}\in \mathrm{Z}$
