Welcome the YT!
Hope others will chime in with their thoughts, but your 4 year old may need to learn that the Yorkie is not one of HIS toys, and encourage him to play with his toys more and not the Yorkie. I would also be sure that the Yorkie has a free/safe place to go to where your children are not allowed to bother him there. Kind of gives them a time out. Kids love dogs, but at that age they have to learn the limits/rules about the Yorkie.
They are both young, but the concern is that the 4 year old does not get bitten and that the Yorkie does not get hurt.
Hang in!
__________________ .   Cali  , and Cali's keeper and staff, Jay No, not a "mini" Yorkie - She loves to motor in her Mini Cooper car  |