Four Puerto Rican nationalists ended a quarter-century of imprisonment and received enthusiatic welcomes here today, still unrepentant for their terrorist attacks on U.S. politicians.

"We have succeeded in our struggle for victory. We are stronger than ever, more committed than ever," said Lolita Lebron, 59, as she was freed this morning from the women's federal prison at Alderson, W. Va.

She and Rafael Cancel Miranda, Irving Flores Rodriguez and Oscar Collazo were granted clemency last week by President Carter, who ordered them freed as "a significant humanitarian gesture," noting that they had served longer prison terms than any other federal inmates.

Lebron led Cancel, 49, and Flores, 55, in the 1954 attack on the House of Representatives in which five congressmen on the floor were wounded by shots from the vistors' gallery. Collazo, 55, had been imprisoned for his attempt to assassinate President Harry Truman in 1950.

The fourth nationalist in the House shootup, Andres Figueroa Cordero, died of cancer in March.

On arrival in Chicago, Lebron seemed simply an older version of the fiery revolutionary who led the House shootup. As supporters raised shouts of Viva Puerto Rico Libre" (Long Live Free Puerto Rico), Lebron said she would return to the island "to light a fire of national liberation."

"We are not here to state our strategies. We would not be revolutionaries if we did so," she said. In a prepared statement Lebron said their release "was done for political expediencey . . . a true gesture of human rights would be the pulling of troops out of Puerto Rico."

Asked if they were proud of their terrorist actions, Lebron said firmly, "Yes! I am proud. I am just like you, a human being."

Collazo said that the aggressors in the events of the past "are not the Puerto Ricans. They are those men who represent American imperialism."

The four made it clear they have no intention of softening their approach when they return Wednesday to the American commonwealth. "A colonial possession does not have political powers," Collazo said. "The only way to have free elections is first to transfer all political power to Puerto Rico."

Addressing himself to Puerto Ricans in the United States, Cancel called them his people as well. "There are no Puerto Ricans here because they like to be here, but because American big money is there [in Puerto Rico] taking everything from us."

Lebron appeared slightly nervous but to have lost none of her intensity over the 25 years in prison. She expressed solidarity in her prepared statement with those she called other revolutionaries: blacks and the new governments in Iran and Nicaragua. "I hope we will also do that someday and clean the world out of tyrants," she said.

After their emotional reunion at the airport, the four led a march of about 1,200 supporters from the heart of Chicago's Hispanic community at Division and Western avenues to a rally at a Congregational church.

From the church steps, the four could be only faintly heard. But the crowd responded with cheers and chants when Lebron declared, "Puerto Rico will be free and Puerto Ricans will liberate it."

Supporters, mostly young, waved Puerto Rican flags and chanted such slogans as "The struggle will come to the people" and "This fist is a socialist fist."

Asked if they still espouse violence, Cancel said before the rally that they never liked it but had to use "any means necessary to resist the enemy." Lebron said, "We did what was necessary in a certain moment of life and that moment may again be now."

Michael Deutsch, a lawyer representing the "Committee to Free the Puerto Rican Prisoners of War" and an organizer of the rally, said President Carter's decision to release the nationalists today "had to do with the nonaligned conference in Cuba." He said the president's purpose was to show that the United States is "not as bad as the other countries were saying."

Earlier in the day, Collazo told reporters as he was leaving prison that he did not consider himself a hero. "I consider myself a man who was doing his duty for his country . . . Truman was a human being. I was not fighting against the man. I was fighting against the system."

Cancel, on leaving the penitentiary at Marion, Ill., reminisced on the 1954 shooting. "It was just something that had to be done," he said. "I don't say I would do it again. Conditions are different today."

On Nov. 1, 1960, Collazo was 36 when he and Griselio Torresola opened fire on Blair House in Washington, where Truman was staying while the White House was being renovated. Truman was not injured, but Collazo's companion and a White House policeman were killed.

Collazo was released from the federal penitenitary at Leavenworth, Kan., Cancel from a federal facility in Marion, Ill., and Rodriguez from a facility in Springfield, Mo.

The four plan to fly to New York Tuesday for an other rally and press conference, before leaving for Puerto Rico Wednesday.