4-month-old babies always crying may be physiological reasons and pathological reasons, such as gastrointestinal discomfort, lack of vitamin D and so on. 1. Physiological reasons: the baby is hungry, thirsty, the environment is relatively cold, etc., or defecation did not change the diaper in a timely manner uncomfortable and other reasons may cause the baby to cry. 2. Pathological causes: (1) gastrointestinal discomfort: as the development of the digestive system of infants and young children has not yet reached full maturity, due to cold, improper diet, etc. may cause gastrointestinal discomfort in infants, abdominal distension, abdominal pain, etc., which leads to infants always crying. (2) Lack of vitamin D: If infants do not supplement or synthesize enough vitamin D in time, the nerve excitability will increase, which may also lead to always crying. There may be other reasons for 4-month-old babies to cry all the time, such as ischemic-hypoxic encephalopathy, etc. It is recommended to consult a doctor in time and follow the doctor’s instructions for further examination, so that the cause can be clarified with the doctor’s help, and targeted treatment or therapy can be carried out.