Four-month-old baby calcium supplement can be through the diet to improve nutrition, more sunshine, oral medication and other ways to supplement calcium. 1. Diet to improve nutrition: generally speaking, the four-month-old baby has not added complementary foods, if the baby is breastfed, the mother can supplement calcium or eat more calcium-containing foods, through the breast milk to indirectly give the baby calcium; if it is a formula-fed, then choose the nutritional formula balanced formula for feeding. 2. Sunbathing: If the weather permits, ensure that your baby spends more than 2 hours outdoors every day; ultraviolet rays can help promote calcium absorption, thus achieving the purpose of calcium supplementation, but you should also pay attention to avoiding exposure to the sun, which can cause damage to the skin. 3. Oral medication: Vitamin D can be supplemented under the guidance of a doctor, generally oral-based. If necessary, follow the doctor’s advice to supplement calcium.