Videotape a converstaion with her. Give her some dolls/stuffed animals and engage her in play-acting what has happened.
I witnessed this in my Ex Wife's family, but it was with the babysitter. The parents ignored it and never did anything about it. The best thing to do is something, talk to her and if need be get social services involved for the child's sake.It will start a family issue if they know you did it, but the priority should be your niece.
As a woman who was molested as a child, I would believe your niece. 4 year olds wouldn't just make things up like that. She had to either experienced it or saw it.
Her imagination isn't just going to come up with that stuff. Get her to talk about it more so that she knows she has someone to go to. I just had a daughter 5 months ago and when I did it triggered anxiety in my mind because when I was 4 I was being molested and wish I would have told someone because now I am in therapy twice a week and on anti anxiety meds because I feel overwhelmed that something might happen to my daughter.At least look into it.
I know I don't know you but please don't take this situation lightly. Just look into it deeper.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.