According to your description, after four years of orthodontic treatment, the first molar turned black due to the oxidation of metal band, and the small black hole on the inner side of the right first molar was what we often call "worm tooth" dental caries, which was caused by poor oral hygiene during orthodontic treatment. Need to be handled in time!

Handling opinions:

1. If the left first molar turns black, you can consider having your dentist do a comprehensive oral cleaning to remove the blackened pigment.

2. Check and fill the "worm teeth" inside the right first molar in time to avoid the aggravation of dental caries.

3. After the above two points are done, re-do the retainer if possible!

I hope my answer can help you! !

Guizhou Anshun Xinyuan dentistry Xie doctor