Figure shows four plates each of area A and separated from one another by distance d. The capacitance between P and Q is

$$\begin{gathered} \mathrm{no} \mathrm{charge} \mathrm{is} \mathrm{stored} \mathrm{in} \mathrm{plate} \mathrm{Q} \mathrm{or} \mathrm{both} \mathrm{the} \mathrm{plates} \mathrm{are} \mathrm{in} \mathrm{the} \mathrm{same} \mathrm{potential} \\ \mathrm{hence} \mathrm{only} \mathrm{two} \mathrm{capacitors} \mathrm{are} \mathrm{formed} \mathrm{and} \mathrm{they} \mathrm{are} \mathrm{in} \mathrm{parallel} \end{gathered}$$$\mathrm{hence} \mathrm{net} \mathrm{capacitance}=\frac{2{\mathrm{\epsilon }}_0\mathrm{A}}{\mathrm{d}}$