It is normal for small puppies to bite, but they need to learn that it's not acceptable. We had this problem with Chloe early on, and we tried the yelping which is based on this method
The Bite stops here, but that didn't do the trick entirely. A man from a yorkie yahoo group, said to try this when she bites:
So she's biting your finger, take your thumb and press down on her snout. I tried that, but that didnt't totally do it, so I finally found my bite correction move that works. When she bites, and I mean EVERY TIME she bites gently close her snout with your hand wrapped around it, and say in a firm, but not loud or mean voice--you know the "I'm the boss" voice..."no bite".
When she goes for white-socked-feet, we clap our hands and say loudly "no bite".
The feet one was really a problem. Even though she was only playing, she'd bite feet hard! So one time, I sprayed her with a single shot of water when she was going for a foot, with a sharp No! and I kid you not, she hasn't done it since. She still likes to play with feet, but she doesn't bite--maybe gentle toothless mouthing, but no biting.
You have to be on it every time though. Chloe is now 4.5 months old, cutting new teeth and everything, and she rarely bites. This is really important because whenever we go places, little girls especially want to pet her and pick her up, and it is a relief to not have to worry about the biting.
hth,