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?