At 4 years old you can pretty much rule out Parvo- this is good. My first thought would be intestinal parasites (worms). You can either take the dog to the vet or take a stool sample to the vet and have this checked out. It is probably not something he ingested at his age unless he is a chronic chewer and gets into things he shouldn't. Is this an issue with him? Any chance he ate something toxic or something that could have irritated his colon? Then there is always a chance of hemagoratic gastroenteritis. He would most likely be lethargic with this and could even be vomiting- but chronic bloody diarrhea is the main culprit. If it is just a little then you can probably safely wait until your vet opens in the morning and just give him water today to rest his stomach. Then tonight or in the morning give him a little boiled chicken and rice and see how he does with that. They can get dehydrated with diarrhea so be sure he drinks plenty of fresh water. Some Pedialyte is fine also. If this was helpful please remember to click the green accept button so we can both get credit for our question and answer.
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