Answer
$x=\pm\frac{\sqrt y}{2}$
When the output is $4$, the input is $\pm 1$.
Work Step by Step
$y=4x^{2}$
Divide by $4$ on both sides of the equation to get
$\frac{y}{4}=x^{2}$
Taking the square root on both sides, we obtain
$x=\pm\sqrt {\frac{y}{4}}=\pm\frac{\sqrt y}{2}$
When $y=4$, $x=\pm\frac{\sqrt 4}{2}=\pm 1$ That is, when the output is $4$, the input is $\pm 1$.
