If after division, one of the charge is $q$ the other charge will be $\left(4-q\right)$. The force between them can be written as,

$F=\frac{Kq\left(4-q\right)}{d^2}$

For the force between them to be maximum,

$$\begin{gathered} \frac{\mathrm{d}F}{\mathrm{d}q}=0 \\ \Rightarrow \frac{K}{d^2}\left[4-2q\right]=0 \end{gathered}$$

$\Rightarrow q=2 \mathrm{\mu C}$