We can use distribution property to multiply the polynomial by the monomial.
$$ \begin{aligned} \left( 16x^2+4x \right) \cdot \color{orangered}{4x} &= 16x^2 \cdot \color{orangered}{4x} + 4x \cdot \color{orangered}{4x} = \\ &= 64x^3+16x^2 \end{aligned} $$