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 .

I reckon it's best to stick to the stipulated date because baby formula is made precisely and contains all the nutrients that newborns need. Nutrients will deteriorate over time and need to be at a certain level for proper infant development. So best not to use baby formula past the manufacturer recommended time.