4 lbs isn't too bad but I wouldn't want a dog smaller then 5 lbs. I too have a 3 year old daughter who like yours loves to take our yorkie for a stroll in the yard
Even though your dog may be good with other dogs, around a yorkie is alot different. You really have to watch your lab because even him sitting on a yorkie could cause a severe injury. My other dog is much smaller then a lab but I need to watch my two carefully together!
On a side note, there really isn't such a thing as a "teacup" just to let you know
