No water is required for a 4mth baby..Breast milk has enough water to fulfil his demand..During the first 6 mths the baby should be exclusively breast fed...no top feeds should be given..Breast feeding has many advantages for the baby as well as for the mother and is the ideal food for the baby fulfilling his energy demands as well as protecting him from infection... Complimentary feeding should be started after 6 mths of age.. There is no ideal weight of a baby...weight gain depends on many factors..most imp birth weight...as long as it is increasing satisfactory and baby is playful..you need not worry..