The excluded volume b is not just equal to the volume occupied by the solid, finite-sized particles, but actually four times the total molecular volume for one mole of a Van der waals' gas. To see this, we must realize that a particle is surrounded by a sphere of radius 2 r (two times the original radius) that is forbidden for the centers of the other particles. If the distance between two particle centers were to be smaller than 2 r, it would mean that the two particles penetrate each other, which, by definition, hard spheres are unable to do.