$y=-4$
A term with $x$ is missing, which means that this term has the coefficient $0$$y=0x-4$
Subtracting is the same as adding the opposite$y=0x+\left( -4 \right)$
Since the equation is written in slope-intercept form, $y=mx+b$, identify the slope of the line as the coefficient next to the variable $x$$\begin{array} { l }y=0x+\left( -4 \right),& m=0\end{array}$
Identify the $y$-intercept of the line as the constant term$\begin{array} { l }y=0x+\left( -4 \right),& m=0,& b=-4\end{array}$
The slope of the line is $m=0$ and the $y$-intercept is $b=-4$$\begin{array} { l }m=0,& b=-4\end{array}$