Answer
$-\frac{\pi}{3}.$
Work Step by Step
Since $\sin\frac{4\pi}{3}=-\frac{\sqrt{2}}{2}$, we have $$\sin^{-1}\left(\sin\frac{4\pi}{3}\right)=\sin^{-1}\left(-\frac{\sqrt{2}}{2}\right)=-\frac{\pi}{3}.$$
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