I've just found this information off a forum it show a possible way to get os4 booting off a hard disk
Here is the solution. First copy the directories Devs, Kickstart, L and Libs from the Moana CD to the root of your MacOS hard disk, ignore the other files in the root of the CD.
Eject the CD, go back to OF and enter “devalias”. This will print all devices from your Mac mini. Locate the entries hd: and cd:. Now make cd: to look like hd: with “devalias cd /pci@f4000000/ata-6@d/disk0” (your Mac mini setup it may differ from mine so change appropriately). Now boot Kickstart with “boot hd:\L\slb” and voila, with some luck you end up in AOS4. Unfortunately this method seems to fail often to load the USB stack. Not the best solutions but hey, it works. The SLB loader is really crap when not booting from a real CD.