what is the reason behind green poo?? my baby is 4mnths old

can be some infection also. how.many times is the baby passing poo ?

Black, Green, Yellow - Your Baby’s Poop Story,  go through this article.

If you find that your baby has passed bright green and frothy poop then it is possible that the baby is taking more feeds of fore milk and this can lead to a weight gain concern. Make sure that you help the baby empty your breasts and also get the hind milk, which will help the baby gain weight adequately

1 or 2 time in a day...

r u giving baby formula milk ???