Find a function S(x) for the surface area of a box that has a square base with length x and a volume of 4m^3

Also the box has no top

Here is what I know,

Volume = x*x*h = 4m^3

Surface Area = x^2 * 4xh
(I'm assuming the top is opposite of the square base)

But now I am totally stumped how to make this a function with x as my only variable. Anyone with any insight on this it would be very much appreciated.

V = x^2*h = 4
S = x^2 + 4xh

x^2*h = 4
h = 4/(x^2)

Substituting into 'S'

S = x^2 + 4x(4/x^2)
S = x^2 + 16/x

'x' is now the only variable. !!!!!