I’ve got a managed LPAR w my D.R. provider and they’ve got to manage their backup schedule fairly closely because they have to logically move the tape drive around for the various AS400′s in our rack. They seem to run into a similar issue.

The limitation may also be with how the tape devices are chained (cabled).

Depending how things are chained it’s going to be Tape Library above the 4 drives, so you’ll have no problem assigning all 4 tape drives to 1 as400.

If you can chain 1 tape library with 1 tape drive to one AS400 then perhaps you can get drives 2 thru 4 to show up on the other systems, but you’ll lose the autoloader functionality. At least with SCSI this would happen.

There’s always VTL.
If you have the DASD allocate virtual tape devices to your system.
You can then run all your backups at the same time and then move the stuff to tape 1 server at a time.