I do not know much about Bass amps but the principles are the same. I'm going to have to guess (bump) for ya. Also ask in the Bass guitar forum - those guys know their stuff.

If you run another 4 ohm cab with you amp then you will be running at 2 ohms total with a total of 300 watts - is the way I see it. I'm guessing the amp can only handle 300 watts total so I'm guessing the extension cab you want to find would 100 watts.

That's just based on the link you provided. Does your owners manual have more info?

Maybe someone else has a better answer. You can also try emailing peavey - they are pretty good about getting back right away.