My 13 year old beagle has cushings disease and has developed colitis. She is on antibiotics, but has not ate in 4 days. I have given her some water, but she is not showing any interest in eating. What can I try to give her to eat?

What did the vet recommend you feed?
Colitis usually requires a high fibre diet so I would not be changing the food at all, but if you like you can add SMALL amounts of something yummy to encourage her to eat.
A tablespoon of yoghurt mixed through the food or a little sardine won't hurt and most dogs love them.
Also try hand feeding her, not something I would normally suggest but in this case you really need to get some food in her system.

Does she seem lethargic? If so try giving her a little glucose syrup to get her sugar levels up, if she is feeling better in herself it also may necourage her to eat.

If that fails though then it's back to the vet, she can't go much longer with no food.