Physics - Capacitor Question with Solution | TestHub

Point $3$ and $4$ are at same potential, hence we can replace the capacitor with a wire.

Capacitors between points $A$ and $3$ are in parallel combination as shown above and their combined capacitance value will be $3C$.

This $3C$ capacitance is in series with $C$.
The equivalent capacitance of the system between points A and B is, 
$C_{\mathrm{AB}}=\frac{3C\times C}{3C+C}=\frac{3C}{4}$
$\Rightarrow C_{\mathrm{AB}}=\frac{3}{4}\times 8 \mathrm{\mu F}=6 \mathrm{\mu F}$