peavey xr500

The only problem you may have is volume. This may not be a problem though because most outputs handle 4 or 8 ohm outputs. As long as you don't actually (physically) wire these speakers with the 4 ohm ones you won't do any damage. Most don't understand that putting two 4 ohm speakers in parrallel (wiring them together to the same output) reduces the speakers resistance to the circuit. In this case instead of an 8 ohm load (which is matched with the outputs impedence) you reduce the speaker load to approx. 2 ohmsm because you have created another path for the output current. This is almost a dead short, sort of like sticking a piece of wire across a 120v outlet in your house. Something will blow or maybe even smoke