How old is your 4lber? I ask this because I thought my 7lber was VERY healthy and doing wonderful. Until she was clipped down at around 9 months old .To my horoor she was way to thin. I could feel her back bones

I very qickly placed Suzi on a high protien canned diet mixed in with her kibble, the same brand she was getting, only it is rated a 6 on the prem. list because of the super protein , and the organic canned is even better.
Since than she has gained , and jsut looks beautifeul, healtheir , and a mighty Yorkie dog. So much stronger and happier now. I do not know , however, With all the hair a yorkie has , underneath a yorkie is so tiny it is scary, and sometimes more underweight than they are ment to be. Not saying it is for yours, but mine was. It was a lesson I learned.
Even now, I feed Suzi, she eats well, and still, she will want more food! She is still being free fed her kibble anytime she wants , and anytime we eat Suzi wants ours too. No matter if she just ate ! I understand how you feel. I have a eater too. Suzi is a high energy dog though. VERY high energy. Perhaps a ADHD Yorkie ,
Maybe free feed your little guy? I don't know though? Is he a high energy yorkie? most likely to run around and burn off all the food he eats? then hungry later? remember too, that such a tiny dog has a very small stomach and can't eat much at one meal. He will be hungry more often, and will need to eat more frequently to make up for such a little tummy and small digestive tract. That is probalbly what is happening to your little one.