WASHINGTON – The national security adviser, Condoleezza Rice, was expected to be nominated as secretary of state, replacing Colin Powell, who was among four Cabinet members to announce their resignations yesterday, adding to a stream of departures that will force President Bush to remake one-third of his Cabinet in the next term.

The 50-year-old political scientist and former provost of Stanford University is one of the president’s closest advisers.

Mr. Powell, a Bronx native who is 67, submitted his letter of resignation Friday, after discussing his pending departure with Mr. Bush for “weeks and months,” he said at a brief press conference. Mr. Powell, who was sometimes perceived as out of step with the president’s foreign policy, emphasized that he had not been nudged out and had always planned to leave after Mr. Bush’s first term. As for what he would do next, he said, “I don’t know.”

In an effusive statement, the president praised Mr. Powell as “one of the great public servants of our time.”

“He is a soldier, a diplomat, a civic leader, a statesman, and a great patriot. I value his friendship. He will be missed,” Mr. Bush said of the former four-star general, who served as the chairman of the Joint Chiefs of Staff in his father’s administration.

Also announcing their departures yesterday were the secretary of energy, Spencer Abraham; the secretary of education, Rod Paige, and the secretary of agriculture, Ann Veneman.

Together with previously announced resignations – those of the attorney general, John Ashcroft, and the commerce secretary, Donald Evans – Mr. Bush faces replacing at least six of his 15 Cabinet secretaries. He has tapped Alberto Gonzales, the White House counsel, to succeed Mr. Ashcroft at the Department of Justice.

The White House emphasized that senior-level departures are a natural part of a second term and that many members, including Mr. Powell, had served longer than average in the highly demanding positions.

As a successor to Mr. Powell, Ms. Rice would benefit from enjoying an “extraordinary closeness” to the president, which Mr. Powell lacked, the director of studies at the Center for Security and Intelligence Studies, Patrick Cronin, said.

“She has huge pluses: smart, elegant, charming, and very loyal to the president,” a foreign-policy expert at the American Enterprise Institute, Michael Ledeen, said.

Several analysts noted, however, that Ms. Rice does not have experience in managing a huge and independent minded bureaucracy.

Ms. Rice’s replacement will be the deputy national security adviser, Stephen Hadley, the Associated Press reported, quoting an unnamed senior official in the administration.

Mr. Powell’s deputy, Richard Armitage, is also expected to resign, the State Department spokesman, Richard Boucher, said.

Despite Mr. Powell’s reputation of being out of sync with some of the administration’s policies, the president yesterday credited him with forging and reinvigorating alliances that are helping in the war on terror. He called Mr. Powell a “key architect” of the Broader Middle East and North Africa Initiative, aimed at promoting democracy in the region.

Early in his tenure, Mr. Powell ran afoul of the president’s policy by stating that the administration would continue a Clinton-era engagement policy with North Korea, only to be overruled publicly by the White House, which said the policy would be reviewed.

Mr. Bush also praised him, however, for defusing a potential crisis when an American Navy reconnaissance plane was intercepted Chinese air space in 2001, and credited the secretary of state with reducing tension on the Indian subcontinent.

The president also said Mr. Powell had turned the world’s attention on the plight of the suffering in Sudan, Liberia, and Haiti, and praised him for “modernizing and strengthening” the Department of State.

After the terrorist attack of September 11, 2001, Mr. Powell succeeded in securing cooperation on intelligence with countries of central Asia and the Middle East, and in gaining basing rights in the buildup to the invasion of Afghanistan.

Mr. Powell is also reputed to have been instrumental, along with Prime Minister Blair of Britain, in convincing Mr. Bush to try to seek U.N. support for toppling Saddam Hussein. He also said of the American commitment to Iraq war, “You break it, you own it.”

He failed, however, to win widespread European support for the Iraq war. His tenure in Foggy Bottom may be most remembered for the forceful presentation he made to the U.N. Security Council that Iraq possessed weapons of mass destruction – weapons that have not been found.

A former Clinton administration official in the departments of State and Defense, Lee Feinstein, who was an adviser to the Democratic presidential nominee, Senator Kerry, called Mr. Powell “the face of moderation in an administration which departed from the foreign policy mainstream.”

“His main legacy ironically will be the brief he carried for the administration at the U.N. Security Council at which he supported flawed intelligence in support of a war that he privately opposed,” Mr. Feinstein said.

In his resignation letter, Mr. Powell wrote, “I am pleased to have been part of a team that launched the global war against terror, liberated the Afghan and Iraqi people, brought the attention of the world to the problem of proliferation, reaffirmed our alliances, adjusted to the post-Cold War world and undertook major initiatives to deal with the problem of poverty and disease in the developing world.”

The White House dismissed speculation that Mr. Powell was pressured out of the Cabinet for disagreeing with Vice President Cheney and the secretary of defense, Donald Rumsfeld. “That’s the typical D.C. speculation game that people like to engage in, no matter how wrong it is,” the press secretary, Scott McClellan, said.

Mr. Powell said he planned to go ahead with scheduled travel and to continue to work for the president’s foreign-policy agenda. He said he would meet with a number of Arab leaders at a conference in Egypt next week.

“And hopefully over the next few weeks I’ll be able to see much – how much potential there is in this new opportunity in the Middle East, with the passing of Chairman Arafat,” Mr. Powell said. The secretary of state brushed off suggestions that his imminent departure would weaken his diplomatic hand abroad.

“I’m still the secretary of state and, as President Bush has made it clear, I operate with his full authority, and so I think that will be recognized by the people that I deal with around the world,” he said.

Mr. Powell was widely praised yesterday.

Britain’s Mr. Blair described him as “a remarkable man and …a good friend to this country over a very long period.” The defense minister of Germany, Peter Struck, called Mr. Powell’s retirement “regrettable” and described him as “a reliable partner in conversation in the area of defense policy,” the Associated Press reported.

The secretary of state said the administration now faces more opportunities than challenges in diplomacy. While acknowledging that the administration had “difficulties” with European nations over the Iraq war, he said: “We are getting rid of those differences and coming together again, as evidenced by the fact that NATO is now undertaking a mission in support of the Iraqi people.”

“We’ve got good relations with China, the best, perhaps, in decades; good relations with India, with Pakistan, with the Russian Federation,” he said. “And all of this, I think, is the result of our foreign-policy efforts over the last four years, under President Bush’s leadership.”

The Cabinet resignations come as the president has announced an aggressive domestic-policy agenda for his second term and faces turmoil abroad, including the insurgency in Iraq, potential nuclear proliferation in Iran and North Korea, strained relations with Europe, and wars in Africa.

“There is a big opportunity and a big risk in having so many Cabinet posts turning over,” the vice president for government relations of the Heritage Foundation, Michael Franc, said.

“There is a big premium on having aggressive personalities in these Cabinet positions who will work overtime to push an aggressive agenda,” he said.

The incoming energy secretary will have the task of pushing the president’s energy plan through Congress, where it has met stiff resistance. The next education secretary will require diplomatic skills in implementing the No Child Left Behind law, which has faced resistance from state and local governments.