4yr child should be16kgs. First thing you should do is to throw away the bottle. Give only 500ml milk per day and increase cereals.

Avoid junk foods,add extra calories to your diet,have 4 to 5 meals rather than large 3...add protein rich foods and healthy fats to diet for e.g..egg white...add fibre rich foods like green leafy veggies,carrots,Apple, etc. Add green tea to your diet .you have to eat more foods high in amino acids,calcium, zinc, iron, copper and magnesium...do regular exercise without failure......consult dietitian or pediatrician for further details.