Can a human live on 4 units of blood?

No, a human cannot survive on only 4 units of blood. The average adult body contains approximately 5 liters (or about 10 pints) of blood. A single unit of blood is equivalent to about 450 milliliters (or about 1 pint). Therefore, 4 units of blood would only amount to about 1.8 liters (or about 3.6 pints) of blood, which is far less than the amount of blood needed to sustain life.