The force of attraction between 2 magnets is inversely proportional to it's distance squared.
That means
[tex]f = \frac{1}{ {r}^{2} } [/tex]
so when the magnets are 4 times apart the resultant force is divided 8 times.
So,
[tex] \frac{f}{8} = \frac{1}{8} f[/tex]
We can see here C=1/8.