If you are fasting for 4 days you need electrolytes or your body will shut down. Sodium, potassium, and magnesium. You can buy fasting salts but they are much cheaper and more balanced to make yourself. As for the shits, yeah that just comes with the territory. My key is to drink your salt water slowly I usually make a liter or so of electrolytes and take the whole day to drink it. Slamming salt water will absolutely gut you. Magnesium is really key for dealing with cramps, I recommend magnesium threonate. Please read the FAQ for info on electrolytes, trying to fast that long without them will be very dangerous.