Before four months old, the digestive system of the infant is very delicate and the body's digestive enzyme is not fully functional yet. Hence, when baby is less than 4 months old, all nutrition that baby needs must either come from breast milk or formula milk powder.
After four to six months, nutrition in the breast milk or milk powder may not be able to support baby's growing need, supplementary food then becomes very necessary. Supplementary food also helps to prepare baby for future weaning.
Four months is not a fixed timing. Parents should observe the behavior of the baby in order to get a precise timing of when to feed baby supplementary food. If your baby's weight stops increasing or he looks hungry even after milk feeding, or he is very interested in the other food on your dinner table; or he is able to grasp other food and feed himself; all these indicate that it is time to give baby supplementary food.
Please also take note that if your baby experiences constipation, bloating, diarrhea after you feed him one supplementary food. You should then consider delay feeding him that supplementary food or change to other supplementary food.