Answer
$31 \ m $
Work Step by Step
$l=\dfrac{\text{Measure of the arc}}{360^{\circ}} \times 2 \pi r ~~~(a)$
or, $l=\dfrac{\text{Measure of the arc}}{360^{\circ}} \times \pi d~~(b)$
Here, $r$ represents radius and $d$ is diameter.
The total length $L$ is given by:
$L=2(L_{inner}+L_{outer})$
So, we have
$L=2[\dfrac{\text{90}}{360^{\circ}} \times 2 \pi (4)+\dfrac{\text{90}}{360^{\circ}} \times 2 \pi (6)]\\=2(2\pi+3)\\=(5\pi)(2)\\ \approx 31 \ m $
