Answer
The required shutter speed is $~~\frac{1}{245}~s$
Work Step by Step
We can see the relationship between intensity, shutter speed, and f-number:
$I \propto \frac{1}{t} \propto \frac{1}{(f-number)^2}$
Then:
$t \propto (f-number)^2$
We can find the required shutter speed $t_2$:
$\frac{t_2}{t_1} = \frac{(f-number_2)^2}{(f-number_1)^2}$
$t_2 = \frac{(f-number_2)^2}{(f-number_1)^2}~(t_1)$
$t_2 = \frac{(4.0)^2}{(5.6)^2}~(1/125~s)$
$t_2 = \frac{1}{245}~s$
The required shutter speed is $~~\frac{1}{245}~s$
