We are glad you are writing in with this sensitive parenting issue. As we experience parenting, we recognize that it may be the toughest but most important job we ever have. As far as the behavior your 4-year-old is engaging in, we suggest you treat it like you would any other behavior that you do not want her to engage in. You have checked with the doctor to make sure everything is physically and medically okay and that is typically the first step to take. Then, just like picking her nose or eating with her fingers instead of a fork or spoon, you want to calmly describe the problem behavior and ask her to stop it. Then redirect her to a positive activity that will keep her hands busy and not allow her to continue what she is doing. Sometimes it is the opposite of what she is doing; for example, you can ask her to go wash her hands or brush her teeth…or to play with a doll or a game like LEGOs that requires the use of her hands.
We are glad you are monitoring her since a 4-year-old child is usually very curious about many things, and they often move from one thing to another very quickly and frequently. We want to keep them safe as well as help them identify what behaviors are okay to use and which ones are not. As parents we are molding and shaping them to make good choices and keep themselves safe. It may be a good idea to also use proper names for body parts so when a doctor or you talk about private parts, she would recognize what that means.
Because she is with her aunt sometimes, let the aunt know how she should respond in a similar manner as you do. This consistency will help your daughter learn that this is not appropriate behavior no matter whose house she is in. There is a great parenting book that you may find helpful for the parenting journey you are on. It is called “Common Sense Parenting of Toddlers and Preschoolers." You can find it online or view it through the boystownpress.org. It has information to help parents be effective and how to teach their children what they need to know to be happy and healthy.