How to stop an 4 month old labrador puppy biting?

First step is to teach HUMANS not to use their hands to tease-play with the puppy. Teaching the pup starts with a quick tug on a short leash with a sharp NO. (No punishment, hitting, or other scolding, just NO once) Then SIT without moving for a minute. Praise him and pat him, and let him play then with a toy that he is allowed to bite and chew on. He needs good strong chew toys like a kong and rope knot.

As soon as he bites again, do this over and over, patiently - tug with no, sit, good dog, chew toy. As long as the humans do their job of keeping their hands away from his face while playing, he will learn very quickly. If you have children you are going to have to explain this carefully to them because the human element isn't usually obvious to them.

This isn't like distracting a toddler. There needs to be that moment of discipline in there, before the praise and the toys he is allowed to chew on. He needs to know the basic commands of no, sit, come, down.