Yep they start falling out around 4 months. They can actually have an overbite during this time too since the mouth will eventually have 42 permanent teeth. In smaller dogs the overbite is not as noticeable.

It is rare to find a tooth. Usually when you see the bleeding one has just been lost and is the best time to find one (again if your lucky). they just all seem to have a way of going to doggie tooth land (very mysterious).

Keep plenty of chew toys around during the first year of pups life since after the major K-9 teeth come in you have erupting molars coming in too and just like a human baby it is discomfort. Mine learned to love chewing ice cubes during this stage! It helps quite a bit!