got exactly the same error on stack of 2 EX4200's with 11.4R1.6:
root@jun_ex4200_stack> request system software nonstop-upgrade /var/tmp/jinstall-ex-4200-xxx
error: Unrecognized command (chassis-control)
warning: The FPC state on LCC may not be ONLINE
I implemented software update by using 'request system software add' command. All went perfectly:
root@jun_ex4200_stack> request system software add jinstall-ex-4200-xxx
Retrieving software images. This process can take several minutes. Please be patient..
Retrieving version and model information from /var/tmp/jinstall-ex-4200-xxx
Checking pending install on fpc1
Checking pending install on fpc0
Pushing bundle to fpc1
Validating on fpc1
Validating on fpc0
Done with validate on all virtual chassis members
fpc1:
WARNING: A reboot is required to install the software
WARNING: Use the 'request system reboot' command immediately
fpc0:
WARNING: A reboot is required to install the software
WARNING: Use the 'request system reboot' command immediately