Handling high pulse current through relay contacts
Let's do the math. You have a \$0.5\ \mu F\$ capacitor charged at 500V.

The energy stored on that capacitor is:

$$E=\frac 1 2 C \cdot V^2 = 62.5\ mJ$$

i.e. very small. 

Even if ALL that energy were discharged on the relay's contacts, the effects would be very limited on a 25-A rated contacts, which are pretty rugged. Beside that, the relay will not experience such energy dissipation, as the SCR will handle the hot switching, and there will be a \$4\ \Omega\$ load.

The mass of the contacts will be in the milligram range. Assuming a very conservative \$ 0.1\ \frac {kJ}{kg \cdot K}\$ heat capacity, and one milligram of contact material, you'll end up in a temperature rise of 625 °C. This is smaller than the melting point of most relay contact materials.

Considered the \$0.1\ \Omega\$ contact resistance, and the load, and assuming zero loss on the SCR, then the energy on the relay will be 1/40, so the temperature rise will be negligible.