Is it possible to have a functional iSCSI cluster with server #4
providing the iSCSI storage via its direct attached Ultra3 SCSI storage
cabinet and in case of failure provide the iSCSI storage via server #3
which would then provide the iSCSI storage via its direct attached Ultra3
SCSI connection to the same storage cabinet?

Server #3 would initially be configured as an iSCSI initiator and be
considered as Node #3 in the cluster but would then become the iSCSI
target in the event of failure of server #4. We would also have servers
#2 and #1 both be participates as node cluster #2 and #1 respectively
attaching to the shared iSCSI storage.