You've noticed this for 4 years - have you seen a doctor?
There are lots of causes for this - some serious, some not, some are just normal variations of skin. Yes, melanoma is a possibility, but that's only one cause.
There is no way someone online can diagnose you. You absolutely need to see a doctor and let them examine you in person, and perhaps run some tests.
https://www.medicalnewstoday.com/articles/320687.php - this lists all the possibilities, but you really need to see a doctor.