bump
This is what it looks like to me: You need the equation of the gray plane in the figure below. To get it, you first calculate a normal which I've shown. Calculate the normal via the cross-product of two vectors which you can calculate emanating from the point (0,4,0). Once you get the normal, then the equation of the plane is
I get:
with
I'm not sure about this. Hopefully, this can get you in the ball-park.