the equation describes a horizontal line with a y-intercept of -4. as the line is horizontal, the slope is 0. The slope is 0 and the y-intercept is -4.
This equation is written in slope-intercept form. This form is y=mx+b, where m is the slope and b is the y-intercept. In this equation, there is no x. This must mean that m is 0 because the only number times x that could make x=0 is 0. If this line was drawn it would be a straight horizontal line, since it is not changing at all the slope is 0.