I assume the myelogram was also a CT scan and there was good evidence of a solid fusion. You still have moderate right foraminal narrowing at C6-7 so that could be a cause as well as degeneration of the C4-5 disc (which can radiate pain to the right neck).
The next step (assuming a solid fusion which is still on the table) is to perform selective nerve root blocks on the right at C6-7, then separately at C4-5. Keep a pain diary (read the hyperlink) and see which one gives you temporary relief. Then, if C6-7 gives relief, have a posterior foraminotomy performed. If C4-5 yields relief, have an ADCF at this level.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.