My baby have not poop since for 4 days. She is currently 4 months old and is exclusively breastfed. Is it something to be worried about? I'm really worried.

Breastfed babies, especially if they have not started solid foods, can easily go two weeks without a poopy diaper once they are 2-3 months old. Breastmilk is exactly what your baby needs, and so there is little waste product left for the baby to poop out.