I have a friend breeder (she has been breeding for a long time and has over 20 dogs) and the vet the works in her kennel said it is not uncommon and unheard of for a yorkie to go for a few days without eating (donnou about 4 days tho), specially when it suffers changes in its routine. If the stools are ok, there is no vomiting but the yorkie is just not eating he or she doesn;t have to be sick or with a problem. But i would definetly try canned soft food...dogs really love it
Keep in mind yorkies are very sensitive animals...another thing, nutrical or other suplements like it, are not a MUST for every dog. None of my dogs ever needed it, and were all healthy...
Just really, be carefull with spoiling him to much and hand feeding, you have to show you are the alpha, also as per feeding, teach him to eat at the right time, i used to remove the food within 1 hour and only offered again at the next meal time, now Mr. Mojo eats withing 10 minutes
About baths, i personally recoment at least 7 or 8 days between baths, that is because excessive bathing may strip the skin of its protective oils and create conditions for skin problems...i know yorkie owner (and breeders) that only bathe their yorkies every 2 weeks (i have done this myself) but it is up to you to decide what is the frequency of baths...