The performance of babies entering the fourth month is usually reflected in several aspects such as social and emotional, language and communication, cognitive and motor development. 1. Social and Emotional: Babies entering the fourth month are able to smile, especially at others. They like to play with others and may cry once play stops, and they can mimic gestures and expressions, such as smiling or frowning. 2. Language and communication: In the area of language and communication, babies begin to babble, expressively babble and mimic the sounds they hear, and cry in different ways to express hunger, pain, or tiredness. 3. Cognition: Cognition includes learning to think and solve problems, babies can express whether they are happy or sad, have emotional resonance, can reach for toys, can use their hands and eyes, such as reaching for toys when they see them, their eyes move from one side of the room to the other with the object, and they can observe people at close distances and recognize familiar people and objects at long distances. 4. Motor development: at this time the baby can be unsupported, steady head up, feet on the ground will automatically press down on the legs, began to roll over, can hold or fling toys, able to swing hanging toys, will put his hand into the mouth. When in the prone position, he or she will prop himself or herself up on his or her elbows. There may be other signs and symptoms in the fourth month of life, so it is advisable to have regular check-ups with the pediatrician.