It's hard not to think of the worst case scenario.
Maybe it was the nake incident she is recalling. Maybe it was the 4 y/o pal that put his "penis in her face". ??
Maybe it was the adult. ??
You may never find out.
I've had my own personal history with this subject and let me just say no matter what in life don't ever leave her alone with a male or a couple that you aren't sure about or even a male family member or neighbor that is a few years older. There are people that are so desperate to get a fix or hormonal 13 year olds or 16 year olds left alone with cute little girls thinking they can get something from that.
As much as it is appalling to all of us, it happens all of the time!
Most of our criminal sex offenders are those that are in the same family or are close family friends/neighbors and not the strangers we fear.
Keep that in mind always.
You'll have to probably find a way to let this go. You may never know the truth and you don't want to blame an adult man when it could have been the 4 y/o son and a combination of other incidents she is combining to make up this story.
The only thing I can suggest is to play a picture game with her. Spread out pictures of family, neighbors, strangers, little boys, the 4 y/o playmate and the adult in question. Then ask her to point to the one that stuck his long, long penis in her face.
You could also call in a police detective to talk to about this and do the picture game in front of. I'd document everything but if you can't determine for sure who it was and what the real story was, you might just have to let this go for your own peace of mind and so as not to further traumatize her.
I'm so sorry any of you had to endure this.

Best of luck!
Follow Mothering