Find $\sin \frac{4 \pi}{3}.$
Converting to degrees,
\[\frac{4 \pi}{3} = \frac{180^\circ}{\pi} \cdot \frac{4 \pi}{3} = 240^\circ.\]Then $\sin 240^\circ = -\sin (240^\circ - 180^\circ) = -\sin 60^\circ = \boxed{-\frac{\sqrt{3}}{2}}.$