\[{\sqrt[4]{162x^{4}y^{8}}}^{4} =\]
\[= \sqrt[4]{3^{4} \cdot 2 \cdot x^{4} \cdot \left( y^{2} \right)^{4}} =\]
\[= 3|x| \cdot y \cdot \sqrt[4]{2} = - 3\sqrt[4]{2}xy^{2}\]