Describe the graph of the equation y = 4. is the equation a function? vertical line; yes horizontal line; no vertical line; no horizontal line; yes

Yes it is a function. It is a horizontal line. 

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