4 months old maybe a little early for the solid food ie cereal you are giving her 6 months or there- abouts is reccomended,in America , as you are still breast feeding check what you are eating/drinking as that can affect babies digestion.Some cereals are fortified with iron also formula is , so read the package iron can make her stool green.It does sound as if its her food, so maybe check out other types and cut back on cereal for the moment . good luck