This dog needs to be seen by a vet ASAP. Dehydration after four days of no fluids is a certainty, and if the dehydration gets too far along, the potassium levels in the body are thrown out of balance and this can cause all sorts of problems, not the least of which is a heart arrhythmia.
A little tidbit of trivia about dehydration - by the time we feel thirsty we are already partially dehydrated. Can you imagine how thirsty you would be if you had had no water for four days? For this girl not to have taken in water for four days she is REALLY sick! I wouldn't wait until morning to go to a vet, at this point I would have her in the car and be on my way to an emergency vet TONIGHT. After four days, no amount of liquid you can get her to drink will be enough to rehydrate her, she needs to be given IV fluids to fill up her cells with water again, and quickly. Then she needs to be thoroughly examined to see what it is that is causing her to be in such pain and she needs to be treated. End of story. I apologize if I sound harsh but at this point it IS an emergency. Please let us know how she is.
Ghilly
Does she stand with her back sort of "hunched" and her tail between her legs?
If so, then this is usually a sign of abdominal pain. So you would have somewhere to start from when you take her to the vet.
Another thought is she could have some spinal or disc problem.
I do think you should take her to be seen by the vet. I know she's 13, but 13 doesn't have to be the end of the world. Just make sure you see a vet who isn't automatically prejudiced -from the start- about her age.
I happen to think (although of course I might be very very wrong here) -that she could have a disc problem, a disc prolapse possibly or maybe an arthritic condition....At the least the vet could give her something to ease her pain. (pain is most likely the reson she won't eat) It may be she will have to rest seriously for a while if there is some damage.
But...and this is very important....she must take on some fluids. 4 days without fluids is serious. She must already be dehydrated. I think the vet will give her intravenous fluids.
While you are waiting to get her to the vet, see if you can encourage her to drink -either pedialyte or water with something like honey or maple syrup added, or even low-salt chicken stock (without onion powder)
But take her to the vet ASAP.
I do hope she is feeling better soon