He threw up four times, and he is not eating for two days

You really need to take your dog to the vet. This could be a lot of things.
What kind of dog is he? How old is he? Is he neutered?
Is he the kind of dog to get into things during walks, or is he left unattended in the yard where he could get into things? Is he the kind of dog to get into things at all?

Did you change his diet, or give him something that's not his regular dog food or treats?

Has he been having diarrhea? Is he drinking? Does he show interest in food, or just turns his nose up? What does the vomit look like (as gross as that sounds).

Sorry for the barrage of questions. You should call your vet. The phonecall is free, and they can advise you on what to do. I would get your dog into the clinic tonight (they will probably want to run bloodwork, or do x-rays, or both). I would try emergency clinics in the area. If there are none, try calling around to different vets. A lot of times vets will do their own on-call emergency work. Again, the phonecall is free, but you sometimes have to pay an extra fee to come in after hours.
If you absolutely cannot take the dog in today, bring him in first thing in the morning. Not eating for this long can be very serious.