K first off flppy isn't IDE...
Make:
Primary master: hard drive
Primary slave: Zip Drive
Secondary master: CDRW
Secondary slave: CDROM
the "slower" zip drive won't hurt your hard drive's performance, some time ago I read a review about that. Unless it's in use the IDE bus will ignore the zip drive, it won't slow the whole channel down.
The only problem with this config is that going CDROM to CDRW copy on the fly isn't a good idea.
Image to your hard drive first, then copy to the CDRW.
My guess is that your CDRW and Zip drive are the lesat used devices, and nomrally you want only one of the two to see heavy use, so separate your hard drive and your CDRom (assumedly the two most used) and your performance should be fine, just having a Zip drive attached to your hard dirve won't hurt it, it will slow it down a bit while the Zip drive is in use, but the hard drive will still work in ATA66 or whatever the drive is.
If you happen to want more than 4 IDE devices get the Promise PCI card.
THat's what I"m doing.