Final answer:
The nurse should check for blood under the client's buttocks to assess for additional lochia flow or a hidden pool, since the fundus is midline and firm, indicating a normal postpartum uterus condition.
Step-by-step explanation:
The appropriate action for the nurse to take when caring for a client who is 4 hours postpartum with a small amount of lochia rubra on the perineal pad is to check for blood under the client's buttock. Since the fundus is midline and firm at the umbilicus, which is normal at this stage, a fundal massage is not indicated. Checking for blood under the buttocks would assess for additional lochia flow or pooled blood that may not be visible on the perineal pad and could indicate an issue such as a postpartum hemorrhage that needs to be addressed.