While 19 of the 23 people initially detained have been released, they all remain suspects in the case.
Three of the women are being held in Stockholm, with the fourth being questioned by police in Dalarna in central Sweden.
Further interrogations of the women are expected to take place on Friday, with prosecutors hoping to reach a decision by the afternoon on whether to request the women be remanded in custody.
A total of 23 people, including 20 women, were arrested on Wednesday on suspicions of being active in a network which distributes images of child pornography.
The suspects are all believed to have used various internet chat sites to make contact with the alleged ringleader, a 42-year-old resident of Borlänge in central Sweden.
According to prosecutor Niclas Elthenius, the man has distributed images to the women.
“They met on different chat forums and then received pictures from the ringleader. They had sought images or expressed a desire to receive these images,” he told the TT news agency.
The man didn’t receive any payment for the pictures, added Elthenius.
“It appears to more of a common interest,” he said.
The women are currently suspected only of possession of child pornography.
“We’re going to try to figure out if they’ve spread the images further, produced images themselves, or received images from other places,” said Elthenius.
The fact the most of the suspects are middle-aged women aged 40 to 60 has flummoxed investigators. Even Interpol has turned its attention towards the case.
“Having women get arrested for child pornography is something unique. On the other hand, material sometimes appears featuring women who carry out sexual assaults on children. But we’ve previously assumed there was a man behind the assault,” said Björn Sellström of the child pornography division of Sweden’s National Investigative Department (Rikskriminalen).
Some of the women also have families, including children who live at home, according to Eltenius.
Most of the suspects were released on Thursday after investigators determined they were no longer able to disrupt the investigation. Interviews had already been carried out and around 30 computers had been confiscated.
According to the prosecutor, some of the suspects have confessed, while others have denied they committed any crime.
Forensic expert experts will carry out an analysis of the computers’ hard drives next week. Depending on what sort of images are found, some of the suspicions may be upgraded to aggravated child pornography offences.
“We know there are some explicit pictures overall. Then we’ll have to see what’s on each person’s computer,” said Eltenius.