Depends on the kinda music yer playin'. Peaveys aren't generally renowned for their tone, but the earlier models like yours can sound pretty good (I've got a Mark III that I use to practice at home and it sounds different, but in a nice way...).
Does the Mark IV have a graphic EQ? I seem to remember that it does... Oh well, anyway, I'd make the eq curve like a gentle arc whose plateau is somewhere in the low-mids, so that the low bass and hi frequencies are rolled off. I dunno what kind of pickups and electrics yer bass has, but I'd try to send a fairly mid-rangey signal out of the bass. But like I said, it all depends on what kind of stuff yer playing.