Oh goodness, yes your puppy is to thin. Puppies while growing should have a nice protective layer of fat under their skin, you should be able to feel the ribs with slight pressure, but not be able to see them from across the room. Bony prominences such as hips and spine should not be palpable either. 4 cups might not be enough right now during the fastest part of growth for male Labrador puppies, sometimes they need as much as 5-6 cups a day to keep up with the demands of a developing body. He needs extra nutrition to grow properly, then when he is full grown you can adjust his diet and take any extra weight off.