thank you!
r = sqrt( x2 + y2 ) y = r·sin(theta)
r - r·sin(theta) = 4
sqrt( x2 + y2 ) - y = 4
sqrt( x2 + y2 ) = y +4
[ sqrt( x2 + y2 ) ]2 = [ y + 4 ]2
x2 + y2 = y2 + 8y + 16
x2 -16 = 8y
8y = x2 -16
y = (1/8)·x2 - 2