You call CPS. You tell them what you heard.
In the case of probable sexual abuse (and, honestly, in what context does a grown man put his "butt" against a 4 year olds where it hurts), you err on the side of caution, which means protecting the child. Quite frankly, she sounds like she's in a situation where she is being traumatized, whether by the boyfriend or someone else is for others to sort out, but, in the meantime, she needs to be protected.
You call CPS. End of story.
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