One of the inmates charged in the fatal prison cell beating of James (Whitey) Bulger, the shrewd and savage organized crime boss who lorded over Boston’s underworld for decades, received an additional sentence of more than four years on Thursday for his role in the attack.

Paul J. DeCologero, a Massachusetts gangster who is serving 25 years behind bars for his activities in the violent DeCologero Crew crime gang, pleaded guilty to an assault charge after prosecutors dropped the more serious counts of first-degree murder and conspiracy to commit first-degree murder — which would have carried a potential sentence of life in prison.

Before the new conviction, Mr. DeCologero, 50, had been eligible for release in December of 2026. Now, that release date has been pushed back to 2031.

Mr. Bulger, who had been suffering from a life-threatening cardiac condition and was dependent on a wheelchair, was found pummeled to death in his cell on the morning of Oct. 30, 2018. It was only hours after he had been transferred from a Florida prison, where he had been serving two consecutive life sentences for his role in 11 murders. He was 89.

Prosecutors have said that Mr. DeCologero had denounced Mr. Bulger as a “snitch” to a fellow inmate and told the inmate that he and another prisoner charged in the case, Fotios Geas, 57, had made plans to kill Mr. Bulger as soon as they saw him enter their unit, located in a federal correction facility in West Virginia known for housing a large number of organized crime convicts.

Prosecutors initially said that Mr. DeCologero and Mr. Geas, a New England Mafia associate who was serving time for his role as an enforcer and hit man, together flogged him in the head with a lock attached to a belt.

But at the hearing on Thursday in U.S. District Court in Clarksburg, W.Va., prosecutors said that Mr. DeCologero had acted only as a lookout and had not struck Mr. Bulger. They said it was Mr. Geas who carried out the bludgeoning, and that Mr. DeCologero had helped him put Mr. Bulger back in his bunk and covered him with bedding. The F.B.I. lab found DNA matching Mr. DeCologero’s on the two blankets that covered Mr. Bulger.

A third inmate, Sean McKinnon, 38, had also been charged with serving as a lookout. In June, he became the first of the group to be sentenced after pleading guilty to lying to federal agents. Mr. McKinnon dodged additional prison time after prosecutors dropped a murder conspiracy charge, and he was credited with the almost two years he spent behind bars after he and the two other men were indicted in the case in 2022.

In May, court records revealed that the three inmates had reached plea deals with the government.

According to Mr. DeCologero’s plea agreement, which was made public on Thursday, he was not debriefed as part of the deal. “He did not provide any information or any substantial assistance to the government in this case or any other case,” the agreement says.

Both Mr. DeCologero and Mr. Geas were originally indicted on charges of first-degree murder, conspiracy to commit first-degree murder and assault resulting in serious bodily injury.

Mr. Geas was also charged with first-degree murder by a federal prisoner serving a life sentence. (He is serving a life sentence for his role in the 2003 killing of a Genovese crime family leader in Springfield, Mass., among other violent crimes.) Mr. Geas’s sentencing in the Bulger case is scheduled for Sept. 6.

The Justice Department said last year that it would not seek the death penalty against Mr. Geas or Mr. DeCologero.

The guilty pleas still have not answered the question of how other inmates were allowed to come into such close proximity to Mr. Bulger.

It was widely known that Mr. Bulger, who had been a fugitive from justice for 16 years until his apprehension in California in 2011, was an F.B.I. informant who for years had funneled information to the agency.

Brian T. Kelly, a former assistant U.S. attorney in Boston who was on the prosecution team that convicted Mr. Bulger in 2013, said in an interview that “incredible bureaucratic buffoonery” had allowed Mr. Bulger to be sent to one of the most dangerous federal prisons and placed in the general population.

“It was the worst possible group to mix him in with because the threat goes up 10 times if you’re a notorious mob informant surrounded by other mobsters,” said Mr. Kelly, who is now at a law firm handling white collar litigation.

“If they can’t figure out that this high-profile prisoner can’t go into the general population with the Massachusetts Mafia, and that one of them was a hit man, then the system has some serious flaws,” he added.

The Justice Department’s inspector general was also critical of the federal Bureau of Prisons in a December 2022 report. The independent watchdog identified a chain of serious administrative mistakes, incompetence and health system failures, and found that officials had upgraded Mr. Bulger’s medical condition solely to clear him for the transfer after he threatened a nurse at the Florida prison where he had been housed.

Yet the inspector general’s report also found that Mr. Bulger, after many months of solitary confinement in Florida, had asked to be assigned to the general population.

In interviews, others familiar with Mr. Bulger and the prison in West Virginia said many questions about the case remain.

Janet Uhlar, who was a juror in Mr. Bulger’s federal trial, began to correspond with him about a month after his conviction. She said she had received dozens of letters from him and they had met in prison on three different occasions for a total of about 15 hours — the last time being about two years before his death.

Ms. Uhlar, who is a nurse, said it was clearly inappropriate to transfer him to West Virginia, given his medical condition.

“It was a blatant, shocking lie that his heart and health had improved,” she said, noting that he was on standing orders for oxygen and nitroglycerin.

Justin Tarovisky, a corrections officer at the West Virginia prison and president of the union local there, was working on the evening that Mr. Bulger arrived at the facility. He said he had been struck by Mr. Bulger’s feeble appearance and was still “shocked” that the warden had allowed Mr. Bulger to be placed in the general population and not in more protective housing.