$\text{At point of return V= 0 i.e K.E = 0}$

From Energy conservation K_i + U_i = K_F + U_f $

$\Rightarrow \frac{1}{2} mv^2 + \frac{kqQ}{\infty} = 0 + \frac{kqQ}{r} $

$ r = \frac{2kqQ}{mv^2} $

$ r \propto \frac{1}{r^2}$

$\text{If velocity is 2v then } r'= \frac{r}{4}$