$x=4y$
$x=\frac{y}{4}\text{, }y\neq 0$

$y=\frac{x}{4}$
$y=4x\text{, }x\neq 0$

$x=\frac{y}{4}$
$x=4y\text{, }y\neq 0$

$y=4x$
$y=\frac{x}{4}\text{, }x\neq 0$