Answer
The charge on capacitor 4 is $45~\mu C$
Work Step by Step
The equivalent capacitance of $C_3$ and $C_4$ is $15~\mu F+15~\mu F = 30~\mu F$
We can find the equivalent capacitance of the four capacitors:
$\frac{1}{C_{eq}} = \frac{1}{30~\mu F}+\frac{1}{30~\mu F}+\frac{1}{30~\mu F}$
$C_{eq} = 10~\mu F$
We can find the charge stored on $C_{eq}$:
$q = C_{eq}~V$
$q = (10~\mu F)(9.0~V)$
$q = 90~\mu C$
Then the total charge stored on $C_3$ and $C_4$ is $90~\mu C$
Since $C_3 = C_4$, both $C_3$ and $C_4$ have the same charge.
The charge on capacitor 4 is $45~\mu C$
