Displacement current is same as conduction current in capacitor. $\begin{aligned}
& X_C=\frac{1}{\omega \mathrm{C}}=\frac{1}{2 \pi \mathrm{fC}} \
& =\frac{1}{2 \pi \times 4 \times 10^3 \times 12 \times 10^{-6}}=3.317 \Omega
\end{aligned}I=\frac{V}{X_C}=\frac{40}{3.317}=12 \mathrm{~A}$