I had a mildly atypical nevus removed 4 years ago, and a tiny freckle has appeared in the scar. i did have a skin check last year. is this cause for concern?

Any new skin lesion that looks different from your other skin lesions or occurs on unusual areas warrants a check by a dermatologist. Since this lesion is on a scar of a prior atypical nevus, it should be checked by your dermatologist or dermatological surgeon. It may only require periodic monitoring but complete removal of atypical nevi is the gold standard.