Answer
Add a 750 pF capacitor in parallel with the original 250 pF capacitor.
Work Step by Step
The energy stored by a capacitor can be expressed as $PE=\frac{1}{2}CV^2$.
In order to store 4 times the energy, using the same battery and the same voltage V, the overall capacitance needs to increase by a factor of 4.
A second capacitor, if it is connected in parallel to the first one, will have its capacitance add to that of the original. Therefore we should connect a capacitor in parallel whose capacitance is 3 times that of the original capacitor. This makes the final capacitance 4 times the original capacitance.
We should add a 750 pF capacitor in parallel with the original 250 pF capacitor.
