Take any value for y. For example y=0. Then z=4-x^2 represents a parabola in the plane y=0 with vertex (0,0,4). Hope you can sketch this parabola. Now take another value for y. Say y=1. Then you have another parabola with the same shape (vertex at (0,1,4)) but on the y=1 plane. Likewise we can choose any value for y. So this represents a parabolic surface,with the vertex as the y axis.