Since she is only 6 months old it is too early to tell how big she will be. Weighing in at 4 pounds when she was 4 months old is not really small. In fact it puts her off the weight chart (
Yorkshire Terrier - Yorkie Puppy Growth/Weight Chart - watch us grow) and gives a guesstimate of well over 6 pounds as adult. If you did not want to breed dogs that are 6-7 lbs because you thught they were too small, how large of female Yorkies were you breeding? You should breed females between 5-7 pounds. Of course bottom line is still a vet check for bone structure, and TESTING. You mentioned not doing that in another thread. I want to encourage you to change your mind. It is really not that expensive. You can get everything you need for probably less than the price of one puppy and then you are doing right by your girl. In addition to the vet check, getting a seasoned expert that shows yorkies to evaluate both dam and sire for conformation is the thing to do. They all look so cute in our eyes and sometimes it takes an unbiased person to really decide if the dog is breeding material. See what the litters have been like in the past 2 generations. What kind of problems have been incurred during delivery? What sorts of health problems have pups shown?
Others have already explained, you can't fatten them up to make weight, but I am guessing that weight is not going to be a problem for you. The mother having a C-section after a few unassisted whelps, is probably not a problem, but best to get the details from the breeder.
Your girl has a cute face in your profile pic. You might want to get some good pics of your dogs stacked and post them for the pros to look at. They will give you some good advice. Even at 6 months they can sometimes tell potential, better at a year though. You won't want to breed her until she has at least her second heat -- usually 18-24 months -- anyway. So you have time for evaluations.