Answer
$\frac{\pi}{3}, \frac{2\pi}{3},\frac{4\pi}{3},\frac{5\pi}{3} $
Work Step by Step
Using trigonometric identities, we have
$4cos^2\theta=1 \longrightarrow cos\theta=\pm\frac{1}{2} \longrightarrow \theta=\frac{\pi}{3}, \pi-\frac{\pi}{3},\pi+\frac{\pi}{3},2\pi-\frac{\pi}{3} \longrightarrow \theta=\frac{\pi}{3}, \frac{2\pi}{3},\frac{4\pi}{3},\frac{5\pi}{3} $