I'm not sure if I can answer your question any better than the last post, but here's my two cents.
It sounds like you're already passing these world-space Vector4 vertices (where the W coordinate is 1) through your view and projection matrices, as expected. As you stated in your last post, you then want to divide XYZ by the resulting W coordinate.
However! As you noted, this will only work when the vertex in question is in front of the camera. At and behind the camera, the results are meaningless. (A vertex at the camera's position will have a W value of 0, division by which is undefined or infinite, and behind the camera will have a negative W.)
So, my first piece of advice is, don't use vertices behind the camera, and this problem won't occur. If you can't avoid this, however, you will have to take a few additional steps to correct it.
In the GPU, before dividing by W, it will clip any polygons that extend behind the near plane (which, in projection space, is Z=0). I think this is what you're going to have to do to handle this scenario.
Let's ditch the quad issue and focus on a triangle instead to simplify. There are four cases to consider.
If 0/3 vertices are behind the camera (none have projection Z < 0), then no problem.
If 3/3 vertices are behind the camera (all have projection Z < 0), then there's no visible polygon anyway.
If 2/3 vertices are behind, we'll have to clip them and work instead with a triangle that ends at the near plane (projection Z = 0).
If 1/3 vertices are behind, we have to clip the triangle, thereby making a quad (two triangles).
In either of these last two cases, we'll have to calculate the locations of the two new vertices. It's pretty easy; just use Vector4.Lerp between the vertex that is in front of the camera to the one behind. The amount of the interpolation will be the ratio of distance to the Z=0 plane from the front vertex to that to the behind vertex. Something like this:
Vector4 atPlane = Vector4.Lerp(front, back, (0 - front.Z) / (back.Z - front.Z));
Do this for each of your unknown vertices - that is, those terminating at this plane - and you can find the vertices of a valid triangle or quad that is entirely in front of the camera. From that point, you divide by W as before. Voila!