A parallel plate capacitor with width \( 4 \, \text{cm} \), length \( 8 \, \text{cm} \) and separation between the plates of \( 4 \, \text{mm} \) is connected to a battery of \( 20 \, \text{V} \). A dielectric slab of dielectric constant \( 5 \) having length \( 1 \, \text{cm} \), width \( 4 \, \text{cm} \) and thickness \( 4 \, \text{mm} \) is inserted between the plates of parallel plate capacitor. The electrostatic energy of this system will be ________ \( \varepsilon_{0} \, \text{J} \).
(Where \( \varepsilon_{0} \) is the permittivity of free space).