If a line is parallel to the x-axis, that means it is a straight horizontal line, with a slope of 0.
The y-value will not change, because it is parallel to the x-axis, and only the x-value will change.
Because y does not change, the equation will simply be y = 4, as the y-value of the given point is 4.
If you want, you can plug in the given point into y = mx + b:
4 = 0*-2 + b
b = 4
Plug in the y-intercept and slope.
y = 0*x + 4
y = 4