It means Data Aquisition Unit nr 4 is off. Try shutting down the Isis computer and restarting it. I ve had this error a few times, either one or multiple DAUs. Sometimes it clears by itself, others I had to restart the system The Isis manual should tell you where DAU nr 4 is locates
I lose data units from time to time, not often. But when I do, sometimes a restart works. Other times after restarting and maintaining the error I disconnect the appropriate data device and plug it back into the system. That always works in the event the restart fails.
