Under Devices in Backup Exec, right-click your device and choose Partition. You will be able to break up your autoloader as you want (Partition 1 - Slots 1 -->4; Partition 2 - Slots 5 --> 8). Once done, open up your backup job, and redirect your jobs to the corresponding slots.
Your retention period will ensure that your oldest tape isn't overwritten until your retention time is reached, upon which is becomes scratch again.