American-born Chilean secret police agent Michael V. Townley was ordered yesterday to serve a minimum of three years and four months in prison - the completion of his deal with the government for his testimony about the 1976 bombing assassination here of former Chilean ambassador Orlando Letelier.

U.S. District Court Judge Barrington D. Parker told Townley that were it not for a plea bargain agreement that limited his sentence, Townley "would be a prime candidate for the maximum penalty" of life in prison.

The tall, bearded Townley had admitted that he helped plot Letelier's death and then planted a bomb under the former diplomat's car that killed Letelier and an aide, Roni K. Moffit.

Yesterday, Parker told Townley that he was "far more culpable" for the murders than the three Cuban exiles Townley helped convict with his testimony at a trial in the federal court here last January and February.

Parker had earlier sentenced two of the Cubans to serve two life terms each - a minimum of 30 years in jail - for their roles in the killings. None of the three had accompanied Townley to Washington to carry out the Sept. 21, 1976, assassination, a mission Townley testified was ordered by the Chilean secret police - once known as DINA.

Townley, who has been imprisoned since Chilean authorities turned him over to the FBI 13 months ago, will be eligible for parole in another 27 months. As part of his plea bargain agreement, the government will recommend that he be paroled at that time although Parker sentenced Townley to serve a maximum of 10 years.

During six days to testimony at the trial of the three Cubans, the 36-year-old Townley recounted in great detail how his DINA superiors wanted Letelier killed and how he carried out the assassination. Letelier, a key official in the deposed regime of the late Chilean Marxist President Salvador Allende, was an arch-critic of the current Chilean military dictatorship of Gen. Augusto Pinochet.

Letelier's killing was and still is the most notorious act of international terrorism ever carried out in Washington.

Before sentencing Townley, Parker asked him whether he still held to his testimony at the Cubans' trial that he had no regrets about killing Letelier. "He was a soldier and I was a soldier," Townley matter-of-factly said at that time, adding that he "very much" regretted the killing of Moffitt.

Townley said yesterday he feels "a great sense of remorse for the death of Ronni Moffitt," but evaded Parker's question about Letelier.

"If I could turn back the clock," Townley said, "I would voice my objections and find some excuse not to follow the orders. I would question the orders. Violence is not the solution to a dispute" Townley said.

The judge then asked Townley about derogatory remarks he made about the judge in a telephone conversation he had with a friend in Chile on Jan. 30 in the midst of the trial. A tape of the conversation was secretly made by Chilean agents in Chile in an effort to discredit Townley's testimony.

In the conversation, Townley said that Parker was "ill-humored and on top of that . . . badly educated." In addition, Townley suggested that he might "ask friends all over the world" to call the judge "and threaten him and get him to withdraw from the case."

An FBI examination of the tape however, showed that while much of the conversation was accurately transcribed, the tape had been doctored and one key sentence was left out of the transcript which indicated that Townley's remarks were said as a joke.

"The allegation of a threat is not so," Townley told Parker yesterday and said that it was a "childish remark, an unfortunate remark." Townley apologized for his comments about Parker's conduct of the trial, and described Parker's treatment of him as "even-handed and fair."

Three former DINA agents, including former DINA boss Juan Manuel Contreras Sepulveda, are awaiting a decision by the Chilean Supreme Court on whether they should be extradited to the U.S. to stand trial for the killings of Letelier and Moffitt.

The Chilean Supreme Court said yesterday it would make a ruling by next Wednesday, but friends of Letelier and Moffitt's husband, Michael, were already predicting that the Chileans will not be tried either here or in Chile.

Saul Landau, who replaced Letelier as director of the Transnational Institute at the Institute for Policy Studies, said that sources in Chile have told him that the Chilean court will rule that Townley's plea bargain arrangement "compromised" his testimony.

Two other Cuban exiles charged with the murders are still at large.