The rule for rewriting rational exponents is: $\displaystyle{{x}}^{{\frac{{1}}{{n}}}}={\sqrt[{{n}}]{{{x}}}}\usepackage{cmbright}\Huge\$

The denominator of the fraction is 4, so this denominator becomes a fourth root.

$\displaystyle{{16}}^{{\frac{{1}}{{4}}}}={\sqrt[{{4}}]{{{16}}}}\usepackage{cmbright}\Huge\$