Answer
$y=3$
Work Step by Step
A line perpendicular to $x=4$ has a slope of $0$. Using $y-y_1=m(x-x_1)$ or the Point-Slope Form, the equation of the line with slope $0$ and passing through $(-2,3)$ is
\begin{array}{l}
y-3=0(x-(-2))
\\\\
y-3=0
\\\\
y=3
.\end{array}