Simplest would possibly be:
CHKTAP DEV(TAP04) VOL(MIMXBK) monmsg ( cpf6700 ) exec( return ) .../* Backup CL follows here... */...
The CHKTAP command will throw a CPF6720, CPF67E2 or some related CPF67xx exception if the volume mounted in TAP04 is not MIMXBK. If you don't want to continue, you can simply execute RETURN.
After the MONMSG command, the backup will run as long as MONMSG doesn't detect an error signal.
You could have it a little better with this:
CHKTAP DEV(TAP04) VOL(MIMXBK) monmsg ( cpf6700 ) exec( do ) .../* Send messages about the tape error... */... return enddo .../* Backup CL follows here... */...
By executing a DO-group, the MONMSG lets you code a series of CL commands before the RETURN. Even if you don't put anything in the DO-group other than RETURN, you should code it as a DO-group. That helps you later when you figure out what you want to do when the volume is wrong. The CPF67xx message will be in the joblog anyway.
A MONMSG command lets you handle errors at different points in your CL program. If there is no error, the CL continues with the next instruction. If you don't RETURN in the MONMSG code or exit in some other way, the CL will still go to the next command in your program.