Answer
$y$ is NOT a function of $x$
Work Step by Step
Take the fourth root of each side of the equation to obtain:
$x=y^4
\\\pm\sqrt[4]{x} = \sqrt[4]{y^4}
\\\pm \sqrt[4]{x} = y$
Note that in the last equation above, for every value of $x$, there corresponds two different value of $y$.
Thus, the $y$ is not a function of $x$.
