If you have 500 watts at 4 ohms how many watts do you have at 8 ohms?

250 watts approximately

I assume you have a constant voltage supply. According to P=V^2/R => P*R=V^2, you have a 44.7 volt supply. If you change Resistance to 8 ohms, P=44.7^2/8 => P=250 watts.