To calculate the drop factor, we need to divide the total volume of red blood cells (450 mL) by the total time (4 hours).

Drop factor = Total volume / Total time

Drop factor = 450 mL / 4 hours

Drop factor = 112.5 mL/hour

However, the drop factor is usually expressed in terms of the number of drops per mL. To convert mL/hour to drops/mL, we need to know the drop size. The drop size is usually given in terms of the number of drops per mL.

Since the drop size is not provided in the question, we cannot calculate the drop factor accurately without this information.