Mr Justice Coulson, who sentenced Juvinal Ferreira for attacking Mark Bridger over a videolink, said the motive for the attack was no mitigation.
"Convicted murderers cannot appoint themselves as unpaid investigators of another crime, no matter how serious. They plainly cannot do that by themselves committing violent crime to do so."
Leeds Crown Court heard that he used a makeshift knife to slash Bridger from temple to chin, severing an artery.
Robin Frieze, defending, said Ferreira did not attack Bridger to enhance his own status in prison, for reward or because he was incited to do it.
"He was under the impression that if he put the complainant in a state of fear then it was more likely it would achieve closure for the family of his victim," Mr Frieze said.
Bridger abducted and murdered five-year-old April Jones on 1 October last year. Her body has never been found.
April Jones' killer, Mark Bridger, was slashed with a knife fashioned out of a razor at Wakefield Prison.
His attacker Juvinal Ferreira, 24, had five years added to his existing term, at Leeds Crown Court today.
The premeditated attack happened three days after Bridger was allowed to associate with other prisoners.
Ferreira and other life prisoners had talked about attacking Bridger to make him reveal the whereabouts of April's body, police were told.
Jonathan Sharp, prosecuting, said: "He said that Bridger's crime had been discussed, and he claimed people had suggested that if Bridger was attacked, rather than say befriended, Bridger would then be more likely to reveal where April Jones's body was."
Bridger needed more than 30 stitches to the face wound, which required surgery under general anaesthetic. He will be scarred for life.
He also suffered a cut to his arm as he tried to defend himself.
April Jones' disappearance and death struck at the very heart of Machynlleth.
Hundreds turned out to search for her when she first went missing, and many more have supported her family throughout their long ordeal.
The community is forever changed by the events of the last twelve months.
The funeral of five-year-old April Jones has taken place in her home town of Machynlleth.
Hundreds of mourners, all wearing pink, have been paying their respects.
The family of April Jones bid an emotional farewell to the five-year-old today, almost a year since the first anniversary of her murder.Read the full story ›
The family of April Jones said a final farewell as the town that shared their grief came to a standstill.
The tiny white coffin carrying her remains was borne through the streets of Machynlleth in a horse-drawn hearse as the community stood in silence or followed behind, all wearing pink in tribute to the schoolgirl.
Paul Davies reports:
Pink floral tributes were placed at the entrance of St Peter's Church by well-wishers, carrying heartfelt messages of sympathy for April Jones and her family.
One stated simply: "Fly high little angel, forever in our hearts, lots of love, Lorna, Boyd, Connie, Ella and Ewan."
Another pink bouquet carried the message: "In loving memory of April. From Nan and Grandad."
The Order of Service pictured a carefree and smiling April Jones on the beach wearing a bright orange top and shorts.
Her devastated mother sobbed audibly in grief as she walked slowly into the funeral service for her murdered daughter.
After the service, the funeral party left the church for a private burial.
The town of Machynlleth will come to standstill today as people pay their last respects to April Jones.
April disappeared when playing outside her home on the Bryn-y-gog estate in Machynlleth on 1st October last year.
She was never seen again and despite one of the biggest police searches ever mounted her body has never been recovered.
Mark Bridger was found guilty of her abduction and murder earlier this year. The 47-year-old paedophile is currently serving a whole life sentence.
A horse-drawn hearse will carry her coffin through the town before a service is held at St. Peter's Church. Local people, many of whom took part in the search to find her, are expected to line the main street.
April's parents, Paul and Coral Jones, have asked people to wear pink - her favourite colour - as a way of remembering her.
Donations made at the funeral of April Jones will be used to sponsor a girl in Africa.
The Rev Kathleen Rogers, who is officiating the ceremony, said donations made at the service will be be used to sponsor a child in Uganda, through the charity World Vision.