4. Let the edge length of a regular octahedron be 1. Then the distance between its two parallel planes is $\qquad$ .
4. $\frac{\sqrt{6}}{3}$.

Let the coordinates of the six vertices of a regular octahedron be $\left( \pm \frac{1}{\sqrt{2}}, 0,0\right),\left(0, \pm \frac{1}{\sqrt{2}}, 0\right),\left(0,0, \pm \frac{1}{\sqrt{2}}\right)$. Then the distance $d$ between the two parallel planes $x+y+z= \pm \frac{1}{\sqrt{2}}$ is $d=\frac{2}{\sqrt{6}}=\frac{\sqrt{6}}{3}$.