You mentioned originally trying to set up /dev/disk0s4 - I'm assuming that was a partition on an internal disk? That should be possible with rawdiskCreator using /dev/disk0 as the disk target (not /dev/disk0s4). You would also need to specify the correct partition number as reported by rawdiskCreator's "print" command (may be different than 4). One thing to note is that Fusion 5 only has support for MBR partition tables (or hybrid GPT and MBR), so rawdiskCreator would not recognize partitions on a disk with only GPT. Support for GPT-partitioned raw disks has been implemented in Fusion 6. It may get integrated it into a future Fusion 5 update, but I can't promise anything.
However, the approach of using a separate "fullDevice" raw disk should still work in Fusion 5, so I'd like to investigate the problem further and figure out why it's happening. The "admin authentication failed or cancelled" message should typically appear only after the authorization prompt is cancelled, but in this case it seems to be fail immediately without even showing the prompt. I'm not sure what would cause this to happen - could you check the system log to see if there's maybe more information? Usually any authorization issues are reported there in more detail. You can also try to trigger the authorization prompt a different way, for example with Help -> Collect Support Information.
A note about permissions - internal disks are typically only accessible by root, and other users (including administrators) don't have read or write privileges. External/removable disks give read and write access to the active user. You can verify the ownership and permissions of all disk devices with: ls -l /dev/disk*
For internal disks, the owner should be root, group should be operator, and the permissions should be rw-r-----, which is read/write for owner, read-only for group, and no access for others. Only root is normally part of the operator group, so even administrator users would require sudo or another way of gaining superuser/root privileges to read or write data to such disks. External disks should have the same permissions, but a different owner and group.
Fusion's raw disk support was mainly developed for Boot Camp partitions on internal disks, so it currently tries to prompt for authorization before trying to open the disk device. So even if the user has appropriate permissions to access the disk (for example if it's external), Fusion will still show the prompt. So you are correct about Fusion mis-reporting that the device is inaccessible - the problem is really that the authorization check failed.
It's possible to turn off the authorization prompt in Fusion, but I'd like to try to figure out what's causing the problem first.