\[\sqrt[8]{x\sqrt[5]{x}} = \sqrt[8]{\sqrt[5]{x^{5}} \cdot \sqrt[5]{x}} =\]
\[= \sqrt[8]{\sqrt[5]{x^{5} \cdot x}} = \sqrt[8]{\sqrt[5]{x^{6}}} =\]
\[= \sqrt[40]{x^{6}} = \sqrt[20]{x^{3}}\]
\[x = \sqrt[3]{4^{20}}:\]
\[\sqrt[20]{\left( \sqrt[3]{4^{20}} \right)^{3}} = \sqrt[20]{4^{20}} = 4.\]