frequent self-gratification

Hello, I have a daughter who is almost 4. In the past 3 moths she has been playing with yourself, spanking the monkey a lot by rubbing her genitalia on a pillow. She usually does not undress to do that. She just lies on her tummy and moves back and forth. This happens daily and sometimes more than once a day. I have been advised to let her do it in private places, so I have taught her to do it in her room. I am somewhat worried b/c this habit is becoming more and more frequent, and steals time from many activities which used to be her favorites. If I try to get her involved in creative play or crafts, she will soon ask "I'm soooo tired...can I have some private time?" (You guessed what it means...). The same happens whenever I say "No" to one of her requests. Lately even when we play or read books she rubs herself with her hands. I' m not sure what to do, I feel stuck. I don't think self-gratification is evil, but it cannot be the only answer to any feeling of boredom or distress.
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