As far as I know scsi.device can't handle ATAPI devices such as a CDROM and that's why we need something like IDEFix or AsimCDFS etc.
To install OS3.5/3.9 you need a CDROM so if you don't have a CDROM the chance you have OS3.5/3.9 installed is slim. ROM3.0 is the most common rom in the A4000 and with it the best WB you can have is 3.1.
Anyway, what you need to know about the A4000 is that it has a very short 5.25" bay so get hold of a short CD drive or you won't be able to fit it inside (also try to make sure that the IDE cable for the CDROM doesn't totaly block all the airholes for the PSU as that is pritty risky and easy to do).
