Answer
$\dfrac{x^3+16}{x}$
Work Step by Step
Let $ y $ be the height of the box.
Surface Area, $ A=x^2+4xy $
$ V=x^2y=4 \implies y=\dfrac{V}{x^2}=\dfrac{4}{x^2}$
Surface Area, $ A=x^2+4xy=x^2+4x \times \dfrac{4}{x^2}$
or, $ A=x^2+\dfrac{16}{x}=\dfrac{x^3+16}{x}$
