The two points share the same y coordinate, which means that the line is horizontal, and this in turn means that the slope is 0.
Since a line written in slope-intercept form is
[tex]y=mx+q[/tex]
In the case of m=0 the equation is
[tex]y=q[/tex]
where the constant [tex]q[/tex] is the common y coordinate shared by all points.
So, in your case, the equation is
[tex]y=4[/tex]
