Every point on this line is of the form (x,4) where x can be any number you want. The y coordinate is fixed at 4 and cannot change. Two points on this line are (0,4) and (1,4). The horizontal line goes through 4 on the vertical y axis.
Why is it a function? Because it is impossible to draw a vertical line through more than one point on the graph of y = 4. The graph passes the vertical line test. Put another way, for any x input, there is exactly one y output.
Side Note: y = 4 is the same as y = 0x+4. Note how its in the form y = mx+b m = 0 is the slope b = 4 is the y intercept
