my son is 4 months old but a premature baby and completely on formula,what should be his ideal weight and feeding quantity?

A. Hai good evening !! Babies gain 400-900 gm per month,by 5-6 months they double their birth weight . The ideal weight of 4 months old should be 3-5.5 kg,as baby is premature baby, if baby has gained 1.5-2 kg since birth then no need to worry. Give exclusive breast milk or formula milk every two hours until 6 months ,burp well.