Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) announced yesterday that he will not run for a fourth Senate term in 1984 and said he will give "serious thought and consideration" to running for president in 1988.

"I have made no secret that I would like to be president," Baker told a news conference in Knoxville, at which he confirmed nearly two weeks of speculation that he would step down, after 18 years in the Senate, to prepare for a presidential race.

"But I think 1984 is Ronald Reagan's year for renomination and reelection," said Baker, reiterating his support for the president. He said he expects Reagan to run for a second term and denied that his own plans were meant as any "signal" of challenge to the White House.

While Baker did not rule out a race for the presidency in 1984 if Reagan does not run again, he said it was his intention "to return to private life at the conclusion of this term," while maintaining "an active interest in Republican politics and public affairs."

There is "life after the Senate," he said, as he prepared to step down at the peak of his legislative powers. "I intend to live that life," he added.

Baker, 57, said he would not leave the Senate or give up the majority leader's job before his term expires. He would almost have to do so to mount a presidential campaign in 1984, in light of his strongly held view that he did poorly in a 1980 bid for the Republican presidential nomination because he remained at his senatorial chores.

Baker's Republican colleagues were quick to express dismay at his impending retirement. Reagan, in a statement released by the White House, called Baker "one of the finest, most skillful majority leaders of this century." Baker's departure from the Senate "will be a great loss to America," the president added.

By stepping down in 1984, Baker, a lawyer before he entered politics, will have time to add to his personal finances and to lay the groundwork, including fund-raising, for a race in 1988. Reagan would no longer be in the presidential picture then because of the two-term constitutional limit.

In a letter to the chairman of the Tennessee Republican Party, Baker also reiterated his long-held "personal conviction that, for me, service in the Senate would not--and should not--be a lifetime career."

Baker, regarded by many Democrats as well as Republicans as the most effective Senate majority leader since Lyndon B. Johnson, has been a major force behind congressional support for Reagan initiatives and, on numerous occasions, has kept the ideologically split GOP majority from unraveling.

But he has also joined other moderate-to-conservative Republicans in pushing Reagan to modify his positions, most recently urging restraint in defense spending, and he barely held the Senate Republican majority together in the fight to pass the administration-backed gasoline tax increase during the lame-duck session late last year.

Since taking over as majority leader in 1981, Baker has ruled largely by consensus, with an affable, low-keyed style that has made him one of the most popular, as well as one of the most powerful, figures on Capitol Hill.

But he faced a potentially serious race for reelection in economically hard-hit Tennessee, where a Democratic senator was reelected last year and Baker's daughter, Cissy, was defeated for a seat in the House of Representatives.

Moreover, the Democrats are expected to make a strong bid to retake control of the Senate in 1984, which would relegate Baker to the role of minority leader, the post he held when he toyed with the idea of quitting the Senate in 1978.

Baker's retirement could trigger a major struggle for succession within GOP ranks, possibly accentuating the ideological divisions that have been at least partly submerged during his leadership.

Possible contenders include Majority Whip Ted Stevens (R-Alaska) and Sens. Paul Laxalt (R-Nev.), James A. McClure (R-Idaho), Robert J. Dole (R-Kan.), Richard G. Lugar (R-Ind.) and Pete V. Domenici (R-N.M.).

The leading contenders for Baker's Senate seat are believed to be Gov. Lamar Alexander (R-Tenn.) and four-term Rep. Albert Gore Jr. (D-Tenn.), both of whom were reelected last November.

However, Alexander would have to step down as governor halfway through his second term, and the Associated Press reported that Republican leaders in Tennessee say he is leaning against doing so, for fear it would cost him votes.