No you CAN NOT use the A3000 version an an A4000!! The busboard is not the same!! The Mediator bridge card is but that is all! You need to update that listing or you will tick off an A4000 user that doesnt have a PCI busboard compatable with it! The edge connectors on the A3000 are not the same and have fewer pins.
Trust me I know this was part of my system this guy told me he was giving a good home to. If you try to put this busboard into an A4000 it will fry your motherboard. If you have a PCI buss thats compatable with a The A4000 mediator card, then it will work with JUST the Mediator card.
FYI to all those looking for a system. Since he sold off my old CSPPC card, and now the Mediator out of my old A3000, I would guess the Mirage Tower is next. People that buy complete systems from people like that and tear them down only to make more money off others through ebay is the exact reason I left the Amiga scene and went Pegasos. If I knew you were going to do this I would have just sold it to someone that wanted a nice Amiga system to use, not to a damn ebay Amiga buy it and sell it for more pirate. Its yours now to do what you wish, but it would have made a great system to someone in need of a complete one. You just may end up making 100% profit off it. Thats the spirit! Thats how to attract people into the Amiga scene and keep the platform alive and going!