My toddler has a habit of twirling/twisting her hair?

My daughter does it as a stress thing. She's almost four and does it when she is in trouble, mad, embarrassed, sad. She doesn't do it enough to cause knots but she twists it in her finger. It does not bother me enough to make her stop but I would suggest every time she does it to move her hand nd tell her not to. Mine does it when she is really really tired to.