4. A circle with a radius of $1 \mathrm{~cm}$, moving arbitrarily within a regular hexagon with a side length of $6 \mathrm{~cm}$ (the circle can be tangent to the sides of the hexagon), the area that the circle cannot reach within the hexagon is $\qquad$ $\mathrm{cm}^{2}$.
4. $2 \sqrt{3}-\pi\left(\mathrm{cm}^{2}\right)$