In the circuit as shown in figure, the effective capacitance between A and B is

The capacitor C4 and Cu are in parallel. Their equivalent capacitance C' = 2 +2 = 4 μF. Now C' is in series with C3. Their equivalent capacitance is given by1C′′=14+14=12  or  C′′=2μFCapacitance C1 and C2 are in series. Their equivalent capacitance C"' = 2 μFThe capacitance a cross AB is given by C=2μF+2μF=4μF.