Final answer:
The nurse's best action would be to closely monitor the drowsy client and notify the healthcare provider immediately.
Step-by-step explanation:
Based on the information provided, the nurse's best action would be to closely monitor the client and notify the healthcare provider immediately. A score of 4 on the Pasero & McCaffery Sedation Scale indicates that the client is moderately sedated and may require intervention to ensure their safety and well-being. Drowsiness and minimal response to physical stimulation suggest a potential adverse reaction to the PCA therapy, which should be promptly addressed by the healthcare team.