your maximum jobs per client to?

Also, check to make sure that you have not selected the "Limit jobs per policy" setting in the policy attributes tab.

Storage unit - have to say max streams per drive- The Maximum streams per drive setting determines the maximum number of concurrent, multiple client backups that NetBackup can multiplex onto a single drive. The range is from 2 to 32.
Schedule - have to say how many on Meida multiplexing-

The Media multiplexing attribute specifies the number of jobs from the schedule that NetBackup can multiplex onto any one drive. Multiplexing sends concurrent backup jobs from one or several clients to a single drive and multiplexes the backups onto the media.

Specify a number from 1 through 32, where 1 specifies no multiplexing.

Policy - The Allow multiple data streams attribute specifies that NetBackup can divide automatic backups for each client into multiple jobs. The directives, scripts, or templates in the backup selection list specify whether each job can back up only a part of the backup selection list. Since the jobs are in separate data streams, they can occur concurrently. - nothing to do with multiplexing.

Master server properties - Global Att. - The Maximum jobs per client property specifies the maximum number of backup and archive jobs that NetBackup clients can perform concurrently. Default: 1 job. , nothing to do with multiplexing.

Master server properties - client att. -The Maximum data streams property specifies the maximum number of jobs that are allowed at one time for each selected client. (This value applies to the number of jobs on the client, even if multistreaming is not used.)  - nothing to do with multiplexing.

There are the only places to manage multiplexing

Do all the jobs start, one writes to tape & the others just queue?

If you have done everything that's been suggested (& in all honesty it looks like you have) then you should see multiplexing - 4 jobs writing to the same tape at the same time, with the remainder q'ing until one of the active jobs finishes.

The only other reason I can think of why you are not seeing multiplexing is if you are expecting 2 or 3 or more different policies/schedules to write to the tape at the same time & they all have different retention periods - by default NB will not mix retention periods on tape (& it's not recommended to do so). Obviously, you would also see this if the schedules that were q'ing were utilising different volume pools.

Can you see in the Activity Monitor the reason why the jobs are q'ing (I presume they are q'ing?) in the "State Details" column or even in the Job Details/Detailed Status? e.g. maximum reached for .... or whatever?

What kind of backups are you doing,  As stated some types of jobs do not allow multiplexing.