Too early. Simply put, a GOOD breeder would never ask you to do this. They would never want you to. Actually, REALLY good breeders do not allow their puppies to leave until 10 weeks. OK breeders allow 8 weeks.
When you have someone just wanting to get rid of puppies, that's the worst. 4 weeks is WAY to young to be on kibble. Seriously. You can pick up the puppy, and it will probably be fine. However, it will have problems that older puppies wouldn't. All puppies bite, but this one will probably be especially mouthy. He might have tummy problems. Potty training will probably be harder. He is too young for shots and baths, so you can't really take him anywhere for socialization or training. He could get sick very easily.
Would I walk away from this? YES. A person taking a puppy away from their mommy's milk at 4 months likely has not taken the best care. They are probably on cheap food. They will have problems, anyway.