Four years ago, in the optimism of a winter noontime sun, Jimmy Carter offered the nation a yardstick by which it could take the measure of his presidency.

"Within us, the people of the United States, there is evident a serious and purposeful rekindling of confidence," Carter said as he stood upon the platform of his inauguration as American's 39th president. "And I join in the hope that when my time as your president has ended, people might say this about our nation . . ."

He went on, that day, to outline far-reaching goals, among them that people would say his presidency had removed the barriers of race and region and religion, strengthened the American family, provided a productive job for all. And he ended with a fervent hope: that his presidency would have "enabled our people to be proud of their own government once again."

But last night, his hopes largely unfulfilled, Jimmy Carter's presidency had come down to one last calling, a message entitled "The President's Farewell Address to the Nation." He sat in a chair in his office, clear of voice and clear of vision, but clearly subdued, as well. He was delivering himself of this message four years earlier than he had hoped, and all too soon he found himself speaking the last lines, "Thank you, fellow citizens, and farewell."

And then the klieg lights were dimmed and America was tuned in to the anchormen and analysts telling them that they had just seen and heard. Barring a last-minute release of the 52 American hostages on his watch, Americans would likely be hearing nothing of official importance from this president again.

Along with an office that is oval and a fanfare of "Hail" all its own, the presidency carries with it the tradition of a farewell address. It has become a poignant and at times powerful instituation of a nation that undergoes its most wrenching change and upheaval while essentially at rest.

George Washington took the occasion of America's first such address to caution his young nation to "steer clearof permanent alliances with any portion of the foreign world"

A century and a half later, Dwight Eisenhower warned in his farewell address that "we must guard against the acquisition of unwarranted influence, whether sought or unsought, by the military-industrial complex."

And last night, Jimmy Carter called on his countrymen to beware of the "single issue groups" and "special interests." He said they "distort our purposes" and he implied that they had, at times, thwarted his efforts to carry out what he called the "broad responsibility to lead" which is incumbent on every president.

There was more, as Carter went on to warn of the peril of nuclear holocaust, the need to safeguard the nation's environment and resources, and the importance of continuing a global emphasis on human rights.

Some of Carter's top advisers, among them Zbigniew Brzezinski, the often-criticized expert on national security, and Stuart Eizenstat, chief of the domestic policy staff, had urged that the speech be much more an outline of the accomplishments of the Carter years, accomplishments of programs that they had worked on for that last four years.

But Carter had gone to Camp David for solitude and reflection shortly after his overwhelming defeat by Ronald Reagan, and he came back with the simple outline of what he wanted to emphasize in his farewell address. And indeed those were the four points he concentrated upon last night.

These were themes, not details of accomplishments; but they were nevertheless themes that his advisers believe will stand his presidency well when compared with whatever the Reagan years may eventually produce, regardless of the unemployment and inflation and interest rates that soared during Carter's years, and the Soviets' move into Afghanistan and to the brink of Poland that tested his presidency.

Last night, as he sat in the Oval Office, this was not the Jimmy Carter that voters had come to know during the harsh and strident days of the 1980 campaign, nor even during those nights of major presidential addresses during the past four years. It was, instead, more like the more conversatonal Jimmy Carter they had seen only once before -- in the fireside chat that he held just two weeks into his presidency.

He had originally intended that fireside chat to be just the first of many such appearances to give the public a quick report on what he was trying to do and how he was trying to do it; but he got bogged down in the detail of trying to master the presidency and never did get around to another such speech until last night's farewell.

"Tomorrow will be two weeks since I became president," he began that night. "I have spent a lot of time deciding how I can be a good president. This talk . . . is one of several steps that I will take to keep in close touch with the people of our country, and to let you know informally about our plans for the coming months."

There was much for the nation to learn about this president in those days, and much for Carter to learn about the presidency. He talked that night, for example, about how his advisers were working on developing a national energy policy. He spoke matter-of-factly about how he had a date-certain for resolving it all: "On April 20, we will have completed the planning for our energy program and will immediately then ask the Congress for its help in enacting comprehensive legislation."

But planning could not be done by precise timetable, he would learn, and Congress had in mind a role somewhat grander than just offering its "help" in passing a Carter program.

Carter is grayer in appearance and wiser in the ways of the presidency now -- just in time to be turning over the key to the office to another man, Ronald Reagan, who is is more than a decade older but not the least bit gray ("Prematurely orange," Gerald Ford once said, describing Reagan's coiffure in less congenial times), and who must now learn the job for himself.

As the president and his advisers prepared for last night, they strove to be businesslike about this moment that was, for each of them, a very personal thing.

"There is a sense of poignancy," said one of the inner circle. "We don't talk about it and neither does he. But there is a sort of quietness and softness about it. He is determined to go out with class, and so we all just take our cues from him."