What could be causing jaw pain/earache a month after 4 impacted wisdom teeth extraction?

Not uncommon. TMJ pain or soreness may occur after the removal of difficult impacted wisdom teeth. In case your pain continues for more than a couple of weeks, you should probably see your doctor for evaluation. In the meantime before your appointment, take NSAID OTC or Tylenol (acetaminophen) to mange the pain or discomfort.
See Oral Surgeon. See the Oral Surgeon who did the extractions. Sometimes the joints and muscles get stressed from opening and stretching and residual soreness results. Also, you may have inflammation remaining, though 1 month later would be unusual. There is always a possibility of remaining or recurring area of infection. Make the appointment and get feeling better. Good Luck.
See Your Doc. Your Doc will need to know if the jaw and/or ear pain just started, or if it has been present the entire month following surgery. A late postop infection could be one cause. Another possibility may be inflammation of the TMJ if you have a small mouth and a complicated or more involved surgical procedure. A follow up with your doc would be highly recommended.
Lot's of things. Too many to list here. Return to the General Dentist or specialist Oral Surgeon who treated you. He/she is responsible for your post-surgical recovery.