The figure shows a parallel-plate capacitor with plates of area $A$ a distance $d$ apart. It was charged to a charge $Q$ and then disconnected from its battery. A slab of dielectric constant $4$, as large as the plates and of thickness $\dfrac{d}{2}$, now lies against the lower plate, and the upper half of the gap is empty. How does the energy stored in the capacitor compare with its value before the slab was put in?

Fig. 1: Side view of a parallel-plate capacitor with two horizontal plates of area A, a distance d apart. The upper plate is marked plus Q and the lower plate minus Q, and no battery is connected. A slab of dielectric constant K equal to 4, as large as the plates and of thickness d over 2, lies against the lower plate. The upper half of the gap is empty. The gap d is marked with a double arrow.