You can convert points from clipping space to camera space by multiplying vertices in clipping space. Clipping space is the view space between camera space and the screen. The x, y, and z of all of the points lie between -1 and 1. X and Y represent the screen position and Z represents the depth. So the vertex (-1, 1, -1) transformed to camera space represents the top left corner sitting on the near clipping plane. Moving Z from -1 to 1 moves the transformed point from the near clipping plane to the far clipping plane.

To transform from clipping space to camera space you first take the inverse of the perspective matrix, I will refer to this matrix as P^{-1}. For each vertex you add a fourth component, w, and set its value to 1. So (-1, 1, -1) becomes (-1, 1, -1, 1). You multiply the resulting 4 dimensional vector by P^{-1}. After multiplying by P^{-1} you divide the entire vector its own w value. The resulting vector will be in camera space. You can then multiply the point by camera world matrix to convert that point to world space.

That is more or less what gluUnProject does, if you want to go this route I would recommend using gluUnProject.

The four points you would have to transform using this method to get a quad to cover the whole screen would be (-1, -1, z) (-1, 1, z) (1, 1, z) (1, -1, z) where z = (zFar + zNear + 2 * zFar * zNear / desiredZ) / (zFar - zNear). desiredZ is how far you want the plane and zNear and zFar are the near and far clipping plane distances for the camera.

That will give you the four corners of the quad that will cover the screen sitting at desiredZ away from the camera.

Although I would just recommend you set both the perspective and modelview matrix to the identity matrix and then render the quad (-1, -1, z) (-1, 1, z) (1, 1, z) (1, -1, z) where z is the same as I have described above. The reason why I would do this is if you try to calculate the quad in world space you are essentially transforming geometry from clipping space to world space then back again. By using the identity for the perspective matrix and modelview matrix you dont actually do any transformation because the geometry is pre transformed.