A 32-year-old woman (O Rh-negative) presents to the emergency department with severe postpartum hemorrhage following vaginal delivery. She has received 4 units of O Rh-positive packed red cells transfused rapidly by the blood bank due to unavailability of O Rh-negative blood. Her hemoglobin is now 7.5 g/dL. On examination, she is tachycardic (HR 118/min) and hypotensive (BP 88/54 mmHg). What is the most appropriate next step regarding further transfusion management?