Energy in our discharging capacitor

U =
Q DV
2
=
C DV2
2
=
Q2
2C

In our case the capacitance is 8 mF and our potential difference was 4000 V. This gives an energy of:
U =
C DV2
2
=
8 x 10-6 * 4000 * 4000
2
= 64 J