You might want to consider a DNA test, my vet told me Wisdom Panel was good http://www.amazon.com/gp/product/B00CAVM9SI/ref=s9_dcbhz_bw_d0_g199_i1_sh
Twelve pounds at 4 months sounds kind of light for a large breed… a veterinarian that examines the dog might be able to tell you the approximate age of the dog.
Most puppies eat 3 small meals per day. Then taper to 1 or 2 meals per day by 7-8 months.
You may find some helpful tips under General Guidelines http://www.homeovet.net/dynamic/php/downloads/dog-c8470f2c75dbe4b683205c3919ee2310/dog_diet_complete.pdf
Also, here http://skeptvet.com/Blog/category/nutrition/
