I would ask her to take the puppy to the vet and have it weighed on their scale and ask for a signed statement of weight on the doctor's letterhead. There is a scale for determining approximate adult size at 8 and 12 weeks but it is a 100% guarantee. Usually you triple the weight at 8 weeks and double it at 12 weeks. I am not sure about 4 weeks but the member with the username femistr has a chart on her website you can refer to. There is no guaratee excpet to buy the yorkie already full grown at 7 months or more. All adult teeth in.
Welcome to Yorkie Talk and good luck in your search.
