Precalculus (10th Edition)

$x=4$.
If a line is vertical, then its equation must be in the form $x=h$ where $h$ is a real number. All points on the line have the same $x$-coordinate. The line is vertical and contains the point $(4,-5)$, the $x$-coordinate of which is $4$. Hence, the equation of the line must be $x=4$.