IDEFix is not needed if you are using AsimCDFS, though it's important to clarify: Do you have Asim 3.1 or Asim 3.10? There's a big difference. Later branches of Asim 3.x definitely have a new driver for the 4000's built-in IDE port, but I'm not sure if 3.1 does.
The reason a new driver is needed is because the IDE driver in ROM - used by Workbench 3.1's default CD0 mountlist - doesn't support the ATAPI standards used by IDE CD drives. So you need a disk-based driver, and Asim has one. IDEFix offers another, but it gets a little messy when you upgrade to OS3.9 with the BoingBags.
The Asim installer should take care of everything for you. When it asks what kind of drive you're using, select ATAPI 1200/4000 (or similar). It'll offer to load SCSI Inquire to find your drive. Odds are it'll be asim_atapi.device, unit 1. Once Asim is installed, make sure your original Commodore CD0 mountlist is removed.
