The main reasons for bone age being 4 years younger are considered to be related to congenital genetic factors, growth hormone deficiency, calcium deficiency, and poor diet and nutrition later in life. 1. Hereditary factors: If the parents have a small bone age, the probability of their children having a small bone age will increase, which is related to the hereditary factors. 2. Growth hormone deficiency: during a child’s growth and development, if there is a lack of growth hormone, it can directly affect the growth of the bones, thus appearing the performance of the bone age of 4 years. 3. Calcium deficiency: children in the growth and development period, the demand for calcium is relatively large. Calcium is the main component of the composition of bones, if there is insufficient calcium intake or excessive calcium loss phenomenon, it will directly affect the growth of bones, thus triggering the phenomenon of bone age is small 4 years old. 4. Acquired dietary nutritional deficiencies: after the birth of a child, if the dietary nutritional intake is insufficient during the period of growth and development, or without balanced supplementation of nutrients, it will directly affect the development of the body, resulting in the phenomenon of bone age of 4 years old. During the child’s growth and development should pay attention to regular physical examination, if the bone age is small 4 years old and other abnormal growth and development phenomenon, need to improve the examination under the guidance of a professional physician, to clarify the cause of the disease and targeted treatment.