Yes, I too have experienced what you are describing. In my case it was due to hyperarousal from excessive stress and anxiety. But, the good news -- I survived. So will you.
Your situation is compounded because of the shift work schedule. My suggestion would be to establish the best and most consistent sleep schedule you can, 7 days a week. Try to minimize napping, avoid caffeine later in the day, and be sure to get some exercise every day if you're not sufficiently tired from your job.
Light control is also important in shift work. You want to ensure you sleep in the darkest room possible, then at your wake time immediately expose yourself to bright light, daylight is best.
Also give yourself a good hour or so before bed to just unwind. Do not allow anything stressful to interrupt this relaxing time to help transition yourself to sleep. A relaxing bath, a relaxing hobby, working with your hands, etc. Try to do this in low light if you can.
If it continues then it's time to see a doc.
4 hrs of sleep per a 24 hr day will affect everyone differently. some can barely function while others can. sounds like you are suffering from sleep starts/jerks. they are medically harmless in the medical field, but i think they are wrong. repeated sleep starts can and have disrupted some ppls sleep to a point theyre afriad to sleep because they are afraid the sleep starts will occur again. this can lead to various mental disorders like depression,anxiety, panic, etc. my opinion would be to get a sleep study done asap