4-month-old babies with blood in stool may be caused by bacterial enteritis, protein allergy, anal fissure and so on. 1. Bacterial enteritis: Salmonella, E. coli, Staphylococcus aureus, etc., can cause the baby mucus blood stools, usually accompanied by foul-smelling stools, may be accompanied by fever, vomiting and other symptoms. 2. Protein allergy: four-month-old babies, if they are mixed feeding or formula feeding, there may be protein allergy phenomenon, at this time the baby will also have blood in the stool. 3. anal fissure: if the baby’s stool is hard, defecation effort, so that the anal opening tear, there will also be a baby with blood in the stool. 4 months baby with blood in the stool there are other possibilities, such as rectal polyps, internal hemorrhoids, etc., it is recommended to consult a doctor in a timely manner to find out the cause, if necessary, with the doctor’s treatment.