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The most common cause of this is a parasite problem. If your puppy has not seen the vet for a fecal exam, you should do so as soon as you are able.
At 4 weeks of age, your puppy should still be nursing and eating small amounts of dog food. Try to encourage him to nurse or bottle feed him for the next couple of weeks.
Let me know if you still have questions.
He is much too young to leave his mother! You need to use mother's replacement milk. I think that you can get it at the pet store or maybe even walmart.
Be sure that you feed him several times a day as well to keep his blood sugar up. This is probably why you are seeing the lethargy and the dog is so quiet.