From what I have seen within my heard, some chins are slow growers but bloom between 10 and 18 months. Your little one is only 4 months so maybe you will have some growth spurts over the year!
The weepy eye and slight overweight can also be caused from candidas (yeast that resides in the body). Mention this if you like to your vet as a possible problem to be checked as well. The teeth should definately be checked. Also the bedding sometimes can be a problem. Pine chips when mixed with urine creates a gas that can be irritating to chins that are sensitive. This is true with aspen, they also can have allergies. Kiln dried pine
has proven to be a good choice or some of the natural paper product beddings are good also.
These are just possible ideas. I hope your chin is found to be in great health!!!