It sounds like you are having a pretty strong cleansing crisis. Toxins are being released into your body faster than they can be eliminated. This is very common in all kinds of fasting protocols. If you are doing water only, then you have chosen the deepest and in some ways most difficult fast. How much preparation have you done to get ready for a fast? If you have not done a whole lot, then you may need to quit and work on a clean diet for a while. You could try Water Fasting one day a week or three days a month to more slowly get the toxins out of your system. Eat unrefined foods, mostly fruits and vegetables and your body will slowly detox and be better prepared for a longer fast in the future.
Four days is pretty impressive, do not feel like a failure if you need to stop the fast. Your body is sending you some pretty serious warnings that its not ready for a prolonged fast-yet.