Is there a way to prolong opened milk formula for more than 3-4 weeks ? I understand that opened milk powder can last 3-4 weeks .

My friends said that opened formula powder can only be kept for 30 days (upon opening). Most keep to that guideline. The nutritional value of the powder may change once opened. Hence, it is probably better to keep to the guideline A few tried to "extend" it by a few days. You can consider getting a smaller can instead.