Reduced urine output in four-month-old babies may be related to immature kidney function, insufficient intake, diarrhea and so on. 1. Immature renal function: small babies with immature renal function may cause slow excretion and reduced urine output. 2. Insufficient intake: if the baby eats less milk, formula-fed babies drink less water, the baby’s body fluid regulation function is weaker, sweating more if you can’t replenish the water in time, there may be a decrease in the amount of urine. 3. diarrhea: the baby if there is diarrhea, will also lose a lot of liquid, there may be symptoms of dehydration, the amount of urine will be reduced. There may also be other reasons for the decrease in urine output of four-month-old babies, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then under the guidance of the doctor’s treatment.