Using Monotouch and OpenTK I am trying to get the screen coordinate of one 3D point. I have my world view projection matrix set up, and OpenGL makes sense of it and projects my 3D model perfectly, but how to use the same matrix to project just one point from 2D to 3D?

I thought I could simply use:

Vector3.Transform(ref input3Dpos, ref matWorldViewProjection, out projected2Dpos);

Then have the projected screen coordinate in projected2DPos. But the resulting Vector4 does not seem to represent the proper projected screen coordinate. And I do not know how to calculate it from there on.