A man convicted of murdering his common-law wife by shooting her in the head with a shotgun was ordered released from prison yesterday by a Prince George's County judge after serving 4 1/2 months of a 10-year sentence.

Judge Audrey E. Melbourne, who had sentenced Herbert Hoover Jackson to 10 years in prison on May 9, suspended yesterday 9 years, 7 months and 15 days of his sentence. She said he had now been punished and keeping him in prison would not serve as any kind of deterrent.

Prosecutor Elias Silverstein angrily attacked Judge Melbourne's ruling. "How can you give a man 4 1/2 months on a second-degree murder charge?" he said. "I've had cases similar to this where the person has gotten life. This sentencing degrades the judicial system."

Judge Melbourne said one of the reasons for her decision to release Jackson, who pleaded guilty to second degree murder in the death of Janice Diane Pinkney, 21, last Dec. 9, was a recent federal ruling ordering 1,000 prisoners released from the Maryland prison system by next spring.

"Most importanly though, the fact that he was contrite influenced me. He was clearly grief stricken and guilt ridden by what he had done.

"But I felt then that he needed to be punished. He needed to think that he was going to jail and the door was going to be locked firmly behind him. So I gave him 10 years then."

But Melbourne said she gave Jackson, 27, the 10-year sentence with the idea that she would seriously consider reducing the sentence later.

"Prior to Jackson's original sentencing a detailed presentence investigation had been done on him, including psychological and psychiatric testing," Melbourne said. "They showed that he had no emotiional problems. In addition, his minister testified very strongly in his favor, saying he was a very active member of the church."

"He had been a model prisoner, he had no thought that he was going to get out quickly and I thought I would be better for him and for his 3-year-old child if he supported her and brought her up instead of her grandparents. That's why I suspended the sentence."

In addition, Melbourne said, the recent court ruling forced her to think practically, as well. "Judges shouldn't have to be concerned with prison space," she said, "that's for legislatures and government leaders to handle. But this is a problem. As a practical matter, a judge has to be concerned with factors like this one."

Jackson was arrested the night on Dec. 9 after he had called his parents and then the fire department, saying he needed help because Pinkney had been shot.

According to his statement made to the police, Jackson had returned to his Bradbury Heights home on the night of Dec. 9 from his job as a truckdriver for the Buffalo Sand and Gravel Co. in Camp Springs.

Walking through the door, he heard the phone ringing and picked it up. It was a finance company calling about a payment, Jackson said, that he had given Pinkney money to pay for.

He said he confronted Pinkney with the unpaid bill and when she failed to produce a receipt an argument started. Jackson went into his daughter's bedroom and got his 20 gauge shotgun from the closet, then went into his own bedroom to get shells.

Jackson said he returned to Pinkney again, asked her for the receipt and when she did not produce it, shot her in the head. Melbourne said that according to later testimony it appeared that Pinkney had not paid the bill because she "was interested in another man." The judge said in a response to a question that it appeared that Pinkney had spent the money on the other man.

State's Attorney Arthur A. Marshall Jr. called the reduced prison term "an inappropriate sentence."

"The sentence is suspended," Melbourne pointed out. "I think this man can be a productive member of society especially since he will get his old job back. But he knows if he gets out of line he'll be back in jail quickly."

Jackson had no previous record, which, Silverston said, fit the patern for those involved in domestic crimes.

"I'm not saying the man needs to be rehabilitated," he said, "because he doesn't. But there is a deterrent factor involved. You can't have people out there believing that if they shoot their wives over an unpaid bill they're only going to jail four months. That's making life much too cheap."