A U.S. court jury convicted Park Policeman Paul Dwain Shepherd yesterday of four counts in connection with a plot to kidnap hotel and restaurant magnate J. Willard Marriott Sr. or Marriott's wife, but acquitted him of three other counts.

The jury, which deliberated for 20 hours over three days, found Shepherd guilty of attempted abduction with the intent to extort money, traveling in interstate commerce to promote kidnaping and extortion, and two counts of soliciting for the commission of a felony.

The U.S. District Court jury in Alexandria acquitted the 44-year-old policeman of extortion, conspiracy to commit kidnapping and conspiracy to commit extortion.

The attempted abduction conviction carries up to a 10 year sentence, while the three other charges of which Shepherd was convicted each carry up to five years prison terms, according to Shepherd's attorneys. Judge J. Cavitt Clarke Jr. schduled sentencing for Dec. 2 and, with the acquiescence of the prosecution, permitted Shepherd to remain free under a $20,000 property bond.

Shepherd, 44, was accused along with his brother, Billy Ray Shepherd, 52, of attempting to kidnap Marriott of Marriott's wife last August in a plot to extort $500,000.

Bill Shepherd, a Calvert County, Md., school maintenance employee, pleaded guilty Monday to one count of conspiracy to use a telephone in interstate commerce to extort money.

Judge Clarke gave Billy Shepherd a five-year suspended sentence plus five years of probation. In return for his plea and for testifying against his brother the prosecution dropped two counts of conspiracy and one each of extortion, abduction and attempted abduction.

Charles Hailey, one of Paul Shepherd's attorneys, said following yesterday's verdict that it has not yet been decided whether to appeal.

During four days of testimony, the policeman's attorneys argued that he as lured into a plot concocted by FBI agents.

Prosecutors said that Shepherd simply was "tired of being a poor cop" and wanted to retire in two or three years with $500,000 in the bank.

To accomplish this, the prosecutors said. Shepherd contemplated kidnaping three person: one of the Marriotts. Redskins Coach George Allen or his wife and former U.S. Transportation Secretary John A. Volpe, owner of a large construction company.

Because he though he and his brother might "screw it up, last July 'Shepherd called Arthur Digges, a convicted felon who is an associate of Spherd brother-in-law, and asked if he or a professional criminal might be interested in helping with the kidnaping, prosecutors said.

Unknown to Shepherd, Digges was an FBI informant. He promptly told the FBI of Shepherd's overture.

Shepherd's version was that he called Digges to negotiate for a stolen tractor he believed Digges could get for him cheaply. Shepherd testified he wanted the tractor as a housewarming gift for his younger brother.

The policeman testified that Digges suggested to him that "the big thing right now is the kidnaping business" and asked Shepherd to name some possible victims. He said he tried to tell Digges he wasn't interested but nevertheless agreed to meet an underworld friend of Digges, Eddie Rossi. Rossi was actually an undercover FBI agent named Edward Robb.

"It was rather exciting, to tell you the truth." Shepherd testified, adding that there was "a lot of mystery" associated with "an underworld figure who was a quote, unquote, hit man."

Shepherd testified he later felt forced into the kidnap plan because the hit man had threatened him and his family. "I'm not going to do nothing" against the Mafia, he said. "I've heard stories about what happens to people who mess with the Mafia."

Assistant U.S. Attorney Justin Williams called Shepherd's version "preposterous" and said that the FBI agent gave him several opportunities to back out of the plan. Shepherd "was the moving force" behind the kidnaping attempt. Williams told the jurors.

Shepherd, who was assigned to D.C. Superior Court as Park Police Liaison officer, was arrested at the court last Aug. 2 after being told by "Rossi" that the kidnaping had been accomplished and then making an extortion call by Marriott's son.