The equivalent capacitance between points A and B in below shown...

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,
${C}_{AB} = \dfrac{3C \times C}{3C + C} = \dfrac{3C}{4}$
$⇒ {C}_{AB} = \dfrac{3}{4} \times 8\mu F = 6\mu F$