Final answer:
Meperidine (Demerol) via IM route is most appropriately used for pain relief during labor when the cervix is dilated less than 4 cm.
Step-by-step explanation:
The drug most appropriately used for pain relief during labor when the cervix is dilated less than 4 cm is Meperidine (Demerol) via IM route. Meperidine belongs to the class of drugs known as opioids, which are commonly used to manage pain. While naloxone (Narcan) is an opioid antagonist used to reverse the effects of opioid overdose and promethazine (Phenergan) is primarily an antihistamine that has sedative properties, they are not typically used for pain relief during labor. Fentanyl (Sublimaze) could be used via the epidural route, but it is generally not the first choice during the early stage of labor before the cervix is dilated to 4 cm or more. Meperidine, meanwhile, has both analgesic properties that reduce pain without loss of consciousness, and in higher doses, can induce drowsiness and loss of consciousness.