Final answer:
The nurse must monitor the patient's respiratory status before administering intravenous morphine sulfate 4 mg since morphine can cause respiratory depression. This is a safety measure to promptly address any adverse effects.
Step-by-step explanation:
Before administering morphine sulfate 4 mg intravenously to a patient for postoperative pain, the nurse should monitor the patient's respiratory status. Morphine, being an opioid, can depress the respiratory system leading to respiratory arrest in severe cases. Monitoring ensures that if the patient exhibits signs of respiratory distress or depression, prompt action can be taken, potentially including the administration of naloxone, an opiate antagonist that reverses the effects of opioids like morphine.