physical chemistry - Why is the van der Waals coefficient b equal to four times the volume of the particle?

In the van der Waals equation of state

$$\left[p + a\left(\frac{n}{V}\right)^2\right](V-nb) = nRT$$

the coefficient $b$ is supposed to represent the volume occupied by the particles. Why then is it equal to four times the volume and not simply the volume?