What happens if the dog is 4 lbs and he eats a small circular ball of milk chocolate?

I would make sure it is milk chocolate. I would watch that dog for symptoms like fatigue, nasue, heavy drooling, and rappid heartbeat. Milk chocolate probably won't cause these symptoms but if it happened to be dark chocolate take that poor dog to an animal emergency room immediatly. An animal emergency room is different from a vet's office. They deal with tragic emergencies that need imediate treatment.