I think all the Mediator boards for the A4000 have Zorro and PCI slots. They may only have one Video Slot, though. Maybe even none. I'm not sure. Check
www.elbox.com for a model that appeals to you. Bear in mind that the 3000T/4000T models only work with original Tower systems, not converted ones.
If you replace your PIV with a Voodoo card, however, you'll lose native passthrough and scandoubling. My guess is that, since you're doing Toaster work, you have another display set up for native work already.
You might be better off just finding a new graphics card, though. What expansions would you use with the mediator?