Have you considered a gfx card? While a little extravagant, especially if all you use your Amiga for is WHDload, they bring the Amiga desktop alive. And unlike an accelerator they are relatively cheap. Picasso II+ would be good? Cybervision 64/3D?
The Picasso II would be alright, but getting a CV64/3D would be a waste, when he can get a CV64 with a build in monitor switcher instead. The CV64 is the ideal card for an A3000.
Will this 4way buffered IDE interface be much faster than the slow mobo IDE controller on the A4000?
It will be just as slow as the onboard IDE controller.