There is no such thing as a 4 year old puppy... I think.
You are correct here but as an only dog we think of her that way.
So as we are not use to 3 pound puppy we keep an eye on him like a new born human baby. Have to feed him twice a day and at 3-4 pounds keep the 20 pound cats from hunting him.
The female puppy, 4 years old, we have is most confused, interesting as she is caught between jealousy of a new dog and her maternal instincts towards this tiny puff ball.
A tough little sucker, he fell off the bed last night with a thump , both of us got up to make sure he was OK. No sign of the Peanut, so we just went back to sleep.
Next Am there he was sleeping behind his new momma with the cats 10 feet away watching him.
Sad that humans do not care for the young as animals do.