What is the potential difference at the centre C ?

Zero

We know that potential is a scalar quantity

$V=\frac{1}{4 \pi \varepsilon_0} \frac{q}{r}$

Where r is the distance between charge and point where potential has to be found.

∴ total potential at center C

$=\frac{1}{4 \pi \varepsilon_0}\left(q_1+q_2+q_3+q_4\right) . \frac{1}{r}$

$=\frac{1}{4 \pi \varepsilon_0 r}(+q-q+q-q)$

= 0