4 months baby stool dark brown with mucus may be caused by functional dyspepsia, intussusception, bacterial dysentery and other diseases, is not normal phenomenon. 1. Functional dyspepsia: improper feeding, Helicobacter pylori infection can lead to functional dyspepsia in babies, babies can cry, bloating, anorexia, vomiting and other symptoms, and the stool can be dark brown and accompanied by mucus. It can be taken orally such as bacillus subtilis bacillus to regulate the intestinal flora, or given triple therapy to treat Helicobacter pylori. 2. Intussusception: If 4-month-old babies start to add complementary foods or milk, it may cause intestinal dysfunction and intussusception due to intestinal maladaptation. In addition, the ileocecal part of the baby is very mobile, which is also prone to cause this disease. The child will have paroxysmal crying and vomiting, and the stool may be dark brown with thin mucus. It is necessary to consult a doctor in time to release the intussusception. 3. Bacterial dysentery: Bacterial dysentery can also lead to mucus stools (i.e., dark brown stools with mucus) in 4-month-old babies, accompanied by fever, diarrhea, crying and other phenomena. It can be treated with oral antibacterial drugs such as cefixime and cefdinir. It is recommended that 4-month-old babies with dark brown stools with mucus should go to the hospital in time to receive treatment as soon as possible after the cause of the disease has been clarified. The above drugs should be used according to the doctor’s instructions.