Before you do anything drastic, you should contact the buyer and find out why she didn’t rate you with a full 5 stars. This would be a very touchy subject, so your tone should not in the least be one of admonishment. Instead, make sure it sounds like you want to know how to better serve her in the future.
While you are at it, you should hint the buyer about the importance of good ratings; some buyers are simply oblivious to their significance to sellers. This knowledge alone may lead to a higher rating in future orders.
For example, something along the lines of:
I have noticed that in your latest review you did not rate my work with a full five stars. Your satisfaction is very important to me, so I would truly appreciate it if you would take a few moments to let me know which part of the service you were not completely satisfied with, in order for me to focus on improving those aspects and better serve you in the future.
Ratings are very important to us sellers, and they are what enable us to keep offering our services in a highly competitive market.
Of course, if she keeps giving you low ratings after this, you may want to stop working with her altogether, but I would advice against burning your bridges. Just turn her down politely, saying something like that you don’t want to work with her if she is not going to be completely satisfied with your service; tell her that there are many skilled sellers here who are sure to be the right fit for her, and wish her the best.