not sure about the mediator. i thought the USB drivers for it only supported the elbox branded usb2 PCI card? afaik, the current version poseidon doesn't support elbox USB cards? i know there was some kinda hoo-har regarding own brand USB cards, mediators, drivers, and over-written hard disk RDB's!
even if i did have an a4000 mediator, i'd use a deneb USB card, as i don't think the mediator uses zorro3 DMA, so i don't think there would be any problem with conflicting DMA xfers.
nice to have a 1600x1200 workbench :-D and if you see a good improvment in 2D performance over the CV64/3D, then go for it. i'm not sure of the driver situation for the radion cards on classic amigas.
personally, i'd only use the mediator for graphics, sound, and ethernet.
USB i'd use a deneb. plus with a deneb, you can boot off USB devices and use keyboard and mouse in early startup if needed..
the scan doubling of standard res screens... well, thats a price of the deal, unless there is a scandoubler released for the A4000 like the one for the A1200