PC = \(\sqrt{(a/2)^2+a^2}\)
PC = \(\sqrt{a^2/4+a^2}\)
PC = \(\frac{\sqrt5a}2\)
Then electric potential
V = \(\frac{kq}{\sqrt5a/2}\) + \(\frac{kq}{\sqrt5a/2}\) + \(\frac{kq}{a/2}\) + \(\frac{kq}{a/2}\)
V = \(\frac{2kq}{\sqrt5a}\) + \(\frac{2kq}{\sqrt5a}\) + \(\frac{2kq}{a}\) + \(\frac{2kq}{a}\)
V = \(\frac{4kq}{\sqrt5a}\) + \(\frac{4kq}{a}\)
V = \(\frac1{4\pi\varepsilon_0}\frac{2q}a(\frac2{\sqrt5}+2)\)