Hi @Rosalie the following thread might answer your concern. https://thelabradorforum.com/threads/is-my-pup-smaller-than-most.25243/#post-381247
Hi @Rosalie, and welcome to the forum. Trying to judge whether a puppy is too thin purely through age and weight alone is very difficult. Labrador puppies are so different, their rate of growth differs, even among their litter-mates. A better way to assess the body condition of your 4-month-old puppy is by how your puppy looks to your eye, this is a good habit to get into. Just stand back and look at your pup. First does your puppy have a healthy shiny and soft coat, bright inquisitive eyes. Secondly feel your puppy. Can you feel or see her ribs clearly? Does your puppy look a bit too rounded? Like a thick blanket around her tummy? Ideally, you should be able to feel her ribs easily, in a puppy of that age, maybe see the last two ribs if she is stretching her body slightly. I would not expect to see the whole rib-cage, spine or pelvis, that would mean she is too thin. Finally you could just pop puppy down to the vets, use this as a socialisation opportunity. Pop her on the scales, and ask the vet nurse on duty what she thinks. I would encourage visiting your vet surgery regularly anyhow, to ensure positive associations with going to the vets, and ask them to pop her weight on their records. You might find this article useful.. labrador-puppy-growth-faq And this thread... how-much-does-your-puppy-weigh.