This section is from NBU admin guide page number - 231, hope this helps you to use only 2 drives out of 4.
Maximum concurrent write drives
The Maximum concurrent write drives setting specifies the number of tape drives that NetBackup can use at one time for jobs to this storage unit. The number of tape drives available is limited to the maximum number of tape drives in the storage device. If a job contains multiple copies, each copy applies toward the Maximum concurrent write drives count.
Select one number for a:
■ storage unit that contains only stand-alone tape drives. Specify a number that is less than or equal to the number of tape drives that are in the storage unit.
■ robot. Specify a number that is less than or equal to the number of tape
drives that attach to the NetBackup media server for the storage unit. Assume you have two stand-alone drives of the same density and specify 1. Both tape drives are available to NetBackup but only one drive can be used for backups. The other tape drive is available for restores and other non-backup operations. (For example, to import, to verify, and to duplicate backups.)
Caution: To specify a Maximum concurrent write drives setting of 0 disables the storage unit.