I have a Mediator PCI 4000Di 3V MK-III Black Edition and have also been having similar issues that many of you are seeing.
The issues seems to be the 'bridgeboard" itself. If I use the daughter board without the "bridge" all my zorro cards work.
With the bridge in and no card (zorro or pci) I can sometimes (oh 5% of the time) get the mediator to boot up. when I put a radeon card in it can get it to boot 5% of the time with only 1-2% of the time were the radeon is recognized.
If I put my 128 meg ZorRam (uses 256 meg of zorro III space) with the bridge and no other cards (pci or zorro) 95% of the time I get a yellow screen, 5% of the time will it boot.
If I put in the zorram and a radeon card with the bridge (either configure with 512 meg space or not) I can get the amiga to boot 5% of the time with like 1% of the time were the radeon is recognized.
Busmastering has even been more of an issue.
Unfortunately I bought this to work with a crescendo 7200, and unfortunately have never seen it work in the A4000. and yes the crescendo does work in my Mac 7200......:-(
For now I gave up and put the Prometheus back in the a4000 with a voodoo 3 and a ethernet card.