[gemy]

Well you all know the obesity problem in humans, dogs kindof reflect that too. Many dogs, large or small are either overweight or obese.

Being a frequent (relatively) visitor to vet's offices, my observation is that a good 70% of the dogs I see I would call overweight, if not obese.

In Yorkies there should be a mild "tuck up" at the loins. Look at your pup from above to see this. Now this is a guideline for an adult dog, and not a puppy of 4mths old.
speak to your vet about your puppy's weight plus their hair loss, is a good first step.