P.E.
Legally, if you think there is a situation where a child is being abused, you have to report it to DCFS (child protections services). They'll take your call anonymously if you want it so.
Although at 4, children can have masturbatory behavior that's considered normal, having the doll as part of the behavior doesn't sound right. She may be trying to act out something that's happening at home or to her.