As of 2019, within 3 decades of Junko Furuta’s death, all four of them have been released.
This year marks the 30th anniversary of Junko Furuta’s death, but surprisingly she is not really remembered in Japan, from what I heard.
I was born and raised in China. Japan as a country can always inflict some complicated feelings among the Chinese. Despite living nearby the location of Nanking Massacre for 5 years, I always try not to judge a nation by its past and sickened individuals, even when some Japanese politicians repeatedly pay their tributes to the Yasukuni Shrine. 
But nothing makes it harder to keep such complicated feelings at bay when I read stories of Junko Furuta’s murder, what has happened to her murderers in the following three decades, and how badly the criminal justice system has failed this innocent 17-year-old who can no longer stand up for herself.
I have written an answer concerning what happened in Junko Furuta’s last days. Alex C. Lee’s answer to What’s the most gruesome murder in history? [ https://qr.ae/T9qOi3 ]
> Junko Furuta was a 17-year-old popular high school girl who was kidnapped and tortured by 4 boys in brutal and unimaginable ways for 44 days before she died. The only reason for the kidnap was that she turned down a guy whom she had no interest in dating.
In the 44 days leading up to her death, here is a non-exhaustive list of what she had undergone:
Held captive and forced to pose as one of the boys’ girlfriend.
Forced to call her parents and tell them that she had run away and was staying safely with friends.
Starved and malnourished.
Raped more than 500 times.
Fed with cockroaches and urine.
Beaten up countless times with bamboo sticks, iron rods and golf clubs.
Forced to masturbate and strip in front of others.
Burned with cigarette lighters and fireworks set off in her ears, mouth & vagina.
Foreign objects inserted into her vagina/anus including bottles, a still lit light-bulb, a cigarette, scissors, and skewers of grilled chicken.
Hands tied to ceiling and body used as a punching bag until her damaged internal organs made blood run from her mouth.
Flammable liquid poured on her feet and legs, then lit on fire.
Forced to sleep outside in December days and sleep inside a freezer.
Hot wax dripped onto her face, eyelids burned by cigarette lighter, left nipple cut and destroyed with pliers.
Stabbed with sewing needles in chest area.
She had been severely damaged before her last breath:
Heavy bleeding from vagina due to scissors.
Nose filled with so much blood that she can only breath through her mouth.
Vomited when tried to drink water (her stomach couldn’t accept it).
Unable to urinate properly.
Injuries were so severe that it took over an hour for her to crawl downstairs and use the bathroom.
Loss of bladder and bowel control.
Eardrums severely damaged.
Extreme reduced brain size.
Unable to move from the ground.
Drastically altered appearance – unable to make out her facial features, severely crippled body with a rotting smell.
During the last few hours she lived, the four boys beat her mutilated body with an iron barbell, put a candle’s flame to her face and eyes, poured lighter fluid onto her legs, arms, face and stomach, and then lit her on fire. This final torture lasted 2 hours.
After Junko Furuta died, the four boys put her body in a 50-gallon drum that they filled with concrete and disposed of it in a park.
The sentence they received is disproportionately lenient, even for juvenile offenders.
The four boys pled guilty to “committing bodily injury that resulted in death”, rather than murder. The court chose to seal the identities of the four boys as they were legally juveniles at the time of the crime.
However, a few journalists discovered their identities and published their names, believing that they did not deserve anonymity.
Hiroshi Miyano, the alleged leader of the crime and also the guy whose miltiple romantic advances were turned down by Junko Furuta, was sentenced to 20 years in prison. He was 18 years old at the time of the crime. He was released in 2009.
Nobuharu Minato, whose home was the location where Junko Furuta was tortured and murdered, was sentenced to 5–9 years in prison. He was 15 years old at the time of the crime.
Yasushi Watanabe received 5–7 years in prison. He was 17 at the time of the crime.
Jō Ogura served 8 years in a juvenile facility before his release in 1999. He was 17 at the time of the crime.
Each of the four perpetrators were protected by special provisions applied to juvenile offenders. During the court proceedings, a couple of people in the courtroom fainted while listening to the details of the brutal rape and torture.
It is believed that the four boys’ affiliation with the Yazuka crime gang contributed to their lenient sentence.
For your reference, here is the court document in Japanese, in case you can read the language. http://www.courts.go.jp/app/files/hanrei_jp/261/020261_hanrei.pdf
(*The house where Junko Furuta was held captive*)
What makes the story sad is that Junko Furuta could have survived, only if the bystander or the police officials make a slight effort.
The four boys were not the only ones who had raped her. Forensic examiners found sperm and pubic hair of other people on Junko Furuta’s remains.
The four perpetrators invited their friends and acquaintances over to Minato’s place, boasting that an “imprisoned woman” was locked up there, available for sex. Even a young girl got invited to Minato’s place and she doodled on Junko’s face. It was estimated that at least 30 others had engaged in sexual activities with her, and at least 100 people knew of her imprisonment.
Nobuharu Minato had a brother, as well as his parents, living in the same house. His brother did nothing except alerting Nobuharu that Furuta might die. His parents did not intervene because they were afraid of their son’s violent tendencies, his affiliation with Yakuza, and losing their reputation in the community.
The police had been alerted twice. The first time, Koichi Ihara, one guy among the many who had been inside Minato’s place, told his brother about the incident. Koichi’s brother subsequently contacted the police, and two officers were dispatched to Minato’s place. However, the two officers left after they were told that there was no girl inside the house, without even doing the basic due diligence. The second time, Junko Furuta managed to reach the telephone and call the police, but she was discovered by Hiroshi Miyano before she could say anything on the phone. The police called back, and Miyano told them that he had dialed it by mistake.
(*The park where Junko Furuta’s body was dumped*)
What makes it even sadder is that the discovery of Junko Furuta’s murder was entirely an accident.
Two weeks after Furuta’s death, Hiroshi Miyano and Jō Ogura were interrogated by the police on a separate gang-rape charge. During Miyano’s interrogation, the police brought up an open murder investigation which Miyano mistakenly believed to be Junko Furuta’s. Led into believing that Jō Ogura had already confessed, Miyano told the police where they had disposed Furuta’s body.
In fact, the open murder investigation was unrelated to Furuta, and that case remains unsolved until today. Hiroshi Miyano simply unwittingly turned himself and his friends in. Despite their unspeakable torture of Junko Furuta, none of them even bothered to fake remorse, and yet they received extremely lenient sentence.
As to where they are now, there isn’t much good news. None of them has shown any signs of remorse. All of them have regained freedom and dated different girls.
After his release in 2009, Hiroshi Miyano changed his name to Yokoyama. However, he is known for being very outspoken about his role in the murder. While talking about it, he displays a playful and exultant attitude, as if it were none of his business. He is also known for his high-end lifestyle, taking friends to barbecue parties and strip clubs, speeding around in a BMW, wearing expensive brand clothing, and bragging about his ties to organized crime and multi-level marketing schemes. 
In 2013, 4 years after his release, Miyano was arrested for suspected fraud. However, there wasn’t sufficient evidence, so he was released without further charges or sentence. At this moment…