Maybe it's not necessarily a whole shift, but "on call"?

If you have to keep the job, just do your best to keep on a schedule.
Take your 4 hour break as "rest time" and tell yourself that if you sleep, good, but if you don't, it's okay because you can sleep when you're shift is over.

If you set your mind to what you know your routine will be, it will be easier. Be strong about it. If you know it's only short term, too, it could be easier.

Your frame of mind is soo important.