AUGUSTA — A former martial arts instructor serving a 40-year federal sentence for child pornography offenses pleaded guilty Monday to four charges of gross sexual assault in which the victims were boys younger than 12 years old.

Wade R. Hoover, 38, of Augusta had been charged with 13 counts of gross sexual assault, but the prosecutor, Deputy District Attorney Paul Cavanaugh, said the state would dismiss the nine remaining counts at the sentencing hearing, which is set for Jan. 27, 2016.

Hoover had previously pleaded not guilty to all the offenses, which occurred between December 2008 and February 2010 in Augusta and in April 2012 in The Forks in Somerset County.

The parents of the victims were in the courtroom to hear Hoover say “guilty” four times. Afterward, a mother of one victim said, “We’re just praying that he gets a longer sentence to keep the children of America safe. It’s the next generation that we need to protect. As long as we keep him off the street, that’s all that matters.”

At Monday’s hearing at the Capital Judicial Center, Cavanaugh said the boy in the Somerset County case had been one of Hoover’s karate students. He said investigators found images and videos of the assaults on electronics seized from Hoover.

He said Hoover was identified as the offender by a number of body tattoos and the boys were identified as well.

“In many videos the children appear to be unconscious,” Cavanaugh said. He said the victims said they have no memory of being assaulted.

In the federal case against Hoover, prosecutors said he drugged the boys before assaulting them.

Cavanaugh told the judge that Hoover acknowledged some of the events and said he “messed with Kennebec victim and fooled around with the Somerset boy.”

Scott Hess, Hoover’s attorney, said Hoover disagreed with some statements that prosecutors said he made, but acknowledged the state had the evidence it claimed.

Cavanaugh said the sentence that could be imposed would be anywhere between 20 years and any term of years because of the young ages of the victims. He also said Hoover would be subject to supervised release rather than probation and would need to register as a lifetime sex offender.

Hoover told the judge that he was confused about how that would work in conjunction with his federal sentence.

Hoover was sentenced in July 2013 in federal court in Bangor to 40 years in prison for producing and possessing child pornography, some of it showing him abusing two boys.

At that time, Hoover told the judge, “I’m pretty much damned in this world and the next.” He also apologized to the victims, saying, “They have the right to hate me. I hate myself.”

Hoover also tried previously to get the state charges dismissed, saying he was being subjected to double jeopardy, and that state prosecutors were using his admissions in federal court to charge him twice for the same offenses.

Hoover was owner and chief instructor at Koshowarrior’s Martial Arts and the United Martial Arts academies in Lewiston, where he taught children as young as age 3, according to Maine State Police. When he was first arrested on charges in October 2013, he was working at the NAMI Maine office in Augusta as a veterans support coordinator.