U.S. Attorney Joseph E. diGenova lambasted yesterday the sentence of a 25-year-old Washington man who received a minimum of five years in prison for fatally beating his girlfriend's 4-year-old son.

"We consider it to be totally inadequate," diGenova said of the sentence given to Terry S. Cain. "We asked for a life sentence . . . . This is a stunningly disappointing sentence. This isn't just child abuse, this is murder."

D.C. Superior Court Judge Robert S. Tignor sentenced Cain on Monday to five to 30 years in prison, saying it was not an easy decision but that Cain had no prior criminal record or history of violence against his own four children or his girlfriend's two children, all of whom lived in a one-bedroom apartment.

The judge also noted that Cain, who pleaded guilty in November to second-degree murder, had attempted to revive Tyrone Champ and called an ambulance. Champ died three days later on Oct. 10 from internal bleeding and extensive brain injuries.

"The quick and easy response would have been to sentence him to the maximum without considering a lot of mitigating circumstances," Cain's attorney, Kathryn Paull Brown, said yesterday. "But the judge refused to take the easy way out. He did what judges are suppose to do. He considered a lot of factors."

Prosecutor Thomas F. McCarthy asked for the maximum sentence of 15 years to life.

Cain did not speak at his sentencing, but in a letter addressed to the court, he described being under severe pressure to support and care for his four children as well as Champ and his brother.

Cain did not describe in detail the events that led up to the Oct. 7 assault.

A psychologist who interviewed Cain wrote in a letter that Cain told him that the beating began after Tyrone jumped on Cain while he was sleeping and "stuck his knee in my face." Cain said he had returned to bed after getting the older children off to school and ironing their clothes.

"I hit him again in the body, was real angry, looked for a belt, hit him again, pushed him, he fell and hit his head," the psychologist wrote, quoting Cain.

Cain told the psychologist he ordered Tyrone to wash his face, but still felt "real mad and vindictive." When the child would not tell him why he jumped on Cain's face, Cain said he hit him again and "something snapped, like a pressure built inside my head."