I am so sorry you are having these pains; persistence of symptoms long-term is a rare problem associated with this surgery. You are correct to seek help with a neurologist, though if no help is forthcoming, you may wish to see an anesthesiologist experienced in pain management. Anesthesia Dolorosa is a general term meaning pain associated with (partial or complete) anesthesia, such as with cut or damaged sensory nerves. This term is often used when neurosurgery for trigeminal neuralgia results in chronic, severe, and unremitting facial pain, though it can be use in the broader sense for other sensory nerves as with your post-surgical situation. I find it unusual that it took three neurologists to say that a nerve "may" have been severed. Without seeing you, any surgeon who performs this operation will tell you that many tiny sensory nerve branches were severed. No question about it!
Any incision through tissues will leave a scar and unavoidably cut sensory nerves which can lead to some degree of numbness or discomfort around or in the area or distribution of adjacent nerve or nerves. Numbness is common, and diminishes over time as nerve branches regrow and heal, but pain is quite uncommon. Severe pain is very uncommon. Pain can result from partially-damaged nerve fibers, regrowth in a disorderly fashion (such as neuroma formation), or other causes. I have performed bicoronal incisions for over 25 years, and have never seen a case of severe pain. Persistent symptoms can be seen, but most patients "get used" to the gradually-diminishing tightness and discomfort. Some may report itching, but this too resolves over time.
After 4 years, your injured sensory nerves have all had time to heal, recover, and any associated scar tissue to soften and mature, so further improvement should not be expected. If you had asked 4 weeks after your operation, any surgeon asked would tell you to wait for maximum healing--at 4 years this is complete.
Your bicoronal incision went through the full thickness of the scalp to your skull, so sensory nerves were indeed cut. The exact placement of the incision (and the exact distribution of your individual nerve fibers) determine exactly where and how many of these nerve branches were cut. I try to choose an incision position that is situated in the "watershed" region between the supraorbital nerves from the eyebrow area radiating backwards towards the crown, and the occipital nerves from the back of the neck radiating forward towards the crown. That way the fewest larger nerve fibers, and the smallest branches, are cut. This results in the least numbness and the least likelihood of post-healing discomfort.
You do not need a test to confirm a diagnosis--your symptoms are enough to make this diagnosis. Sensory nerves were cut--they had to be in order to do your surgery! Treatment is the proper consideration now. Local anesthesia injection can confirm the location (by relief) of the offending nerve fiber or fibers, and options exist to give you long-term improvement, if not complete relief. If your neurologist consultants are no longer able to assist you, ask for referral to a pain specialist who can do test injections to assess relief, recommend TENS or other options, or even nerve ablation (if an exact site of problem can be identified). Good luck!