BackupExec only recognize two of my four drives tape library

What does the Windows Device Manager say, as well as the front control panel of the library?

Additionally, you are licensed with 3 additional LEO licenses right?  Library Expansion Options.

Without LEO licenses BE can't use more than one drive at the same time. BUT CAN RECOGNIZE, if OS detects.

This library has an ethernet interface, what do you see if you connect to the webpage of the library ?

Also check cabling (and scsi ID's/terminaters), make sure all drives are connected properly !