My baby is less than 4 kg and she is 1 month 18 days old .is it ok or not?

Don't worry. I think it's a great weight if your baby's birth weight was around 3kgs. Let me tell you my story. My boy weighed 2.6 kgs at birth. Lost weight in the initial few days. Came up to 2.5kgs by 1 month. Took few more days to reach birthweight. I used to be very worried. Used to run to paediatrician. But I never opted formula milk for him. Now he is 9 months old, weighs 7 kgs. He is active, happy, achieving his milestones each month. He is already standing on his own. That's all matters. Not every baby will be chubby. They don't have to be.