child nipple stimulation

My 4 year old daughter is stimulating (pinching and rubbing) her right nipple with her left hand multiple times each day.  I am aware of the chemical release of oxytocin which is the result of nipple stimulation, and understand its similarities to child masterbation (masturbation) and I am considering previous advice on that topic.

My concern is for the long-term physical health and appearance of her nipple and breast.  Her right nipple is always larger, more inflamed, and much redder than her left nipple, regardless of how many hours it has been since she last had a stimulation session?

My questions:  How much is too much?  What are some possible long-term consequences if this behavior continues?  What can I do to help her reduce or mitigate this habit?