There are many O2's on eBay at the moment, ranging from $10 to $100. Unfortunately, A4000's go for up to tenfold that amount, so I wouldn't get my hopes if I were you..
By all means, they are awesome workstations. I've been working with O2's on several occations and they were superior on 3D graphics back in the 90's.
Good-looking piece of hardware, too.
Shoot, now I want one.
