I just wanted to reiterate what some have already said;
1. You want to avoid any breeder that purposley breeds for Tinies, Teacups, Micro's, etc. that is just a few words they will use. Also avoid buying from a breeder that will breed her tiny female. Standard size of a Yorkie is 5-7 lbs. and MOST REPUTABLE breeders will not breed a female under 5 lbs. MOST that do are breeding for the tinies and the $.
2. Also, just because the mother and father are 4 lbs. or under DOES NOT mean that your baby will not hit larger sometimes even larger then standard. You do not know what genes are in the background. My suggestion to you is if you ONLY WANT a 4 lbs or under dog then wait it out and look for an adult say 10 months or older. USUALLY (Although some still can have a growth spurt) their growing is done by a year old and slows greatly say after 6 - 7 months so I think 10 months or older is a good age for you to shoot for.
3. You don't have to spend big bucks to get what you want. Patience. Patience is the key. You mentioned an older dog. I've seen quite a few over the past year in rescue that were tiny. In a lot of instances their owner dies or is put in a nursing home and the family hands the dog over to rescue. In some instances this dog in rescue came from a puppymill and was used to breed just what you are looking for "Tiny Yorkies." Yorkies as are all popular breeds are big money to byb or puppymills. Although rare you may also find a young yorkie pup in rescue too, but as said you really do not know what the weight is as NO ONE can guarantee the weight of a puppy. They can only guestimate and a GOOD BREEDER will tell you that.
Best of luck to you.
Elaine |