Black stools in 4-month-olds may be related to the addition of complementary foods, iron supplements and upper gastrointestinal bleeding, and the specific situation varies from person to person. If a 4-month-old has added complementary foods, there is often a relationship between the complementary foods added and the color of the stool. For example, the color of the stool usually deepens and becomes darker after adding spinach and black sesame seed paste. When children take iron supplements orally, the iron will combine with hydrogen sulfide in the gastrointestinal tract, which will further cause the feces to turn black in color. This usually requires no special treatment, and the color of the stool will return to normal when the iron is stopped. Upper gastrointestinal bleeding refers to bleeding caused by lesions in the esophagus, stomach, and duodenum above the flexor ligament, with typical symptoms such as vomiting blood and black stools. Clinical manifestations such as palpitations, irritability and pallor are often seen in cases of massive bleeding. If you find that your toddler has frequent black stools, you should consult a doctor without delay so as not to delay your condition. Pay attention to rest on weekdays and take a reasonable diet.