I'm sure ur puppy bites when he's in a playful mood. I also own a 4 month old Lab and I know when he's likely to bite anyone. When he comes back from a play session, he'll franatically run from one corner of the house to another and if I dare to stop or if anyone comes on his way, he'll severly bite him/her. Having noticed this recurrent behaviour, I nsure when he comes back from play session, he's properly kept on leash for an hour or so.
Sometimes, when I'm on bed or chair, he'll come and start to take my finger on his mouth. With no delays, I immediately get up and leave the room. When I return back after 2minutes, I find him well mannered. This reaction of mine conveys a message to the puppy that his bitting is not liked by me and it leads to a loss of his play mate. The next time, I find him quietly laying down besides my feet!!!
Also, a bored dog may resort to biting to break his boredom. Daily walk, helps the dog physically, but it doesn't mentally stimulates him. If u want to train the dog to not to bite, you must train him to do other things. A mentally busy puppy will never feel the need to bite his master.