Take him to the vets immediately, he may have some sort of blockage, if I had a dog who had been sick for 4 days it would be off to the vet, and I'm not a great lover of vets. Dogs pick up and swallow all sorts of things, mostly they work there way through the system without causing any harm, but occasionally they get stuck somewhere and cause a blockage, if he is still pooing this is unlikely, but get him checked out anyway.