Four equal charges $q$ each are placed at four corners of a square of a side $a$ each. Work done in carrying a charge $-q$ from its center to infinity is

Potential at center $O$ of the square due to four equal charges $q$ at four corners

$\mathrm{Each} \mathrm{of} \mathrm{the} \mathrm{charge} \mathrm{is} \mathrm{at} \mathrm{a} \mathrm{distance} \mathrm{of} \frac{\mathrm{a}}{\sqrt{2}} \mathrm{from} \mathrm{the} \mathrm{centre}$

$$\begin{gathered} V=\frac{4q}{4{\mathrm{\pi \epsilon }}_0\left(\mathrm{a}\sqrt{2}\right)/2}=\frac{\sqrt{2}q}{{\mathrm{\pi \epsilon }}_0\mathrm{a}} \\ {W}_{0\to \infty }=-W_{\infty \to 0}=-\left[-qV\right]=\frac{\sqrt{2}{q}^2}{{\mathrm{\pi \epsilon }}_0\mathrm{a}} \end{gathered}$$