Have had a "buffalo hump" for about at least 4 years. Don't have any other symptoms at all. Am a little overweight. Any reason for concern?

Probably not ..... .... but this unusual deposition of fat at the neck and upper back is associated with Cushing's syndrome. Named after a famous doctor from the 19th century, it occurs when there is too much circulating cortisol/cortisone in the bloodstream. The body's adrenal glands may produce too much or a person may be taking/using cortisone drugs for a prescribed treatment. You should be thoroughly tested.
Concern. Yes, a relatively new "buffalo hump" strongly suggests you have a weight problem that needs attention.