1. What is complementary food for four-month-old babies? For the first six months of a baby\’s life, breast milk or formula is the only source of nutrition. But as babies grow older, around four months, they need more nutrients to meet the needs of physical development. At this time, complementary foods need to be introduced. Complementary food for four-month-old babies refers to the first solid food introduced to your baby. What kind of complementary food is suitable for babies aged two to four months? At about four months, the baby\’s digestion is still very fragile and cannot accept foods that are too complex or too exciting.