4u is usually 4 units, and 4 units of blood is usually about 480 milliliters. Red blood cells account for about 40%-50% of all blood, 1 unit of blood is separated from 200 milliliters of whole blood from red blood cells, along with other anticoagulants, calcium ions, etc., so the volume of 1 unit of blood is about 120 milliliters, 4 units of blood is generally about 480 milliliters. Blood transfusion is commonly used in the clinic for patients with acute blood loss, iron deficiency anemia, aplastic anemia, acute leukemia and other hematologic disorders with anemia. When transfused to patients with various types of anemia, the patient’s hemoglobin can be raised to a level of about 5-6 grams with the transfusion of 1 unit of red blood cells. According to the degree of anemia of the patient, according to the number of units of blood transfusion of the patient, it can be roughly deduced that after the transfusion, the patient’s hemoglobin raised grams, which is also a common method to determine the effect of blood transfusion of the patient. In addition, before transfusion, it is necessary to conduct cross-matching, blood type test, evaluation of liver function, infection program and other tests under the guidance of the doctor, and select the appropriate blood type for transfusion.