Why am I having excruciating tooth pain after 4 months of root canal treatment?

Please go back. To see the provider who did the root canal to explore the reason for this pain.
Call dentist. I'm sorry you're having such pains after a root canal and that is not the norm. You need to see the dentist who did the root canal and have it check it out and to hear what your symptoms are and then the problem can be resolved.
Root canal pain. Bad news. I'd get the tooth scanned with 3d x ray ( cbct) and evaluated for fracture. For sure there is chronic infection if still hurts. Lots of systemic worries besides pain. Look into spirochetosis. Bad critters can grow around those areas of teeth.
Not 100% Sometimes root canal treatment doesn't work. Go back to dentist who did root canal. If it was a general dentist, get an opinion from an endodontist. You may either need re-treatment of root canal, or extraction.