A federal judge yesterday cut in half the 4 1/2-year prison term of former George Washington University professor Dr. Murdock Head, accepting arguments that the judge who sentenced Head had intended to grant him an early release.

Yesterday's decision by District Judge James C. Cacheris, which Cacheris said he reached after consulting his fellow federal judges in Virginia, means Head will be eligible for parole from his 1981 bribery conspiracy conviction as early as November.

Head, 59, creator of the sprawling, wooded Airlie conference center in Warrenton, Va., and its affiliated foundation that figured prominently in his case, was scheduled to be considered for parole in September 1984.

Frank W. Dunham Jr., one of Head's lawyers, said he notified his client of the decision by telephone and said that Head was "extremely pleased." Head has been incarcerated in a federal correctional facility at Maxwell Air Force Base in Alabama since March.

Head was convicted in 1979 of conspiring to bribe two powerful House Democrats in exchange for a steady flow of government grants to his Airlie organization. That conviction was overturned by a federal appeals court, but prosecutors won a second conviction two years ago in U.S. District Court in Alexandria.

Cacheris said in the order released yesterday he believed Head--a tall, trim fitness buff who holds degrees in law, medicine and dentistry and who founded GW's department of medicine and public affairs--has "unique talents" that could be put to use performing community service in Fauquier County, where Airlie is located.

The judge ordered that, once paroled, Head serve a special four-year period of supervised probation and perform 20 hours a week of community service work. Cacheris said he was allowing the arrangement in spite of prosecutors' claims that it would be "illusory and insubstantial" in Head's case.

Cacheris also said he felt "duty bound" to grant Head's request for a reduced sentence because of remarks made at sentencing by the late Judge Oren R. Lewis, who raised the possibility of a shorter prison term if Head agreed to take part in community programs.

Cacheris said he was "not impressed" by prosecutors' arguments that "the failure of the defendant to admit his guilt should be held against him."

The judge's decision followed a campaign of support for Head that included letters of praise ranging from former Defense Secretary Melvin R. Laird to the mayor of Warrenton and the Fauquier County volunteer fire and rescue service.

Federal probation officers also were informed by Air Force officials at Maxwell that Head has been active in several programs for military personnel and inmates at the base, according to defense lawyer Dunham.

Dunham said Head has instituted aerobics classes for inmates and has helped process Air Force applicants to a community college on the base, arranged seminars and suggested guest speakers for the school since he began his sentence six months ago.

Head has proposed that he instruct Fauquier County's volunteer rescue squad in trauma techniques, develop seminars for police in forensic medicine, aid authorities in fighting substance abuse and help the county sheriff with health problems at the Fauquier jail, according to Dunham.

Dunham said Head, who severed his ties to George Washington after his conviction, has a standing offer for a job in continuing education at the university once he is released from prison. The position would be "staff, not faculty," Dunham said.

He said the question of whether Head might resume an active role in running Airlie "hasn't been decided yet."