He was turned in by the girl’s mother and grandparents after German authorities took the unusual step of going public with images of the young victim yesterday in a bid to identify her.
Before midnight the same day both victim and abuser had been identified and a man is now in custody.
Authorities are set to release more information about the case during the course of the day.
The photos of the female victim were released with an appeal for information about who she is.
Police said she was subjected to “serious sexual abuse” between October last year and July this year.
Vile pictures of her suffering were uploaded to the Dark Net by her tormentor – images said to have sickened hardened lawmen.
Chief prosecutor Georg Ungefuk from the Central Office for Combating Cyber Crime said of the perpetrator: “He is a German citizen. The man is being questioned. The girl is also German and four-years-old.
“He was arrested in an apartment by state police.”
According to Bild the arrest took place in Lower Saxony.
Herr Ungefuk took the decision to release the images of the girl on a Monday stating: “This is the last measure to identify the perpetrator. We only fell back on this as all other possibilities have not led to finding out who she is.
“We assume that the child is further exposed to the perpetrator’s access,” Herr Ungefuk added. She is now in the care of child psychiatrists.
A judge travelled to the jail where he is being held – and quizzed by sex crime detectives – to formally charge him.