While U.S. attention in the hemisphere has shifted to the South Atlantic, four volatile countries of Central America are still undergoing the turbulent changes that made them so much the focus of Reagan administration concern.

Developments of the past six weeks, many of them unexpected, have created new dangers as well as new opportunities for a U.S. policy of "drawing the line" against revolutionary changes in the hemisphere. Official Washington, however, is only beginning to come to grips with altered circumstances.

This sense of rapid change, which is outrunning the limits of previously established U.S. policy and even of much of the domestic controversy about it, is among the stongest impressions arising from an 18-day visit to El Salvador, Guatemala, Honduras and Nicaragua.

As viewed from the region, both the administration's central political-economic enterprise, the Caribbean Basin Initiative (CBI), and the central demand of its congressional critics, a negotiated settlement of the Salvadoran civil war, appear inadequate and even largely irrelevant to conditions now at hand.

For a first-time visitor, two underlying facts stand out about this band of countries, which also includes Costa Rica, Panama and Belize, between North and South America.

First there is geography, which makes the Central American nations far more important to the United States than they would be if situated elsewhere.

To go to Vietnam in the years of U.S. engagement took a day across the Pacific via the best jet plane connections, and to go to conflict sites in the Middle East takes almost as long via airline transfers in Western Europe. Guatemala City, however, is only two hours and 12 minutes from Miami via small airline jet, and flights within the region take 20 to 30 minutes at most.

A private American living in Guatemala said, "Between here and Texas there is only Mexico, and it's shaky." An experienced U.S. official in the area said, "If this were Tanzania or someplace, I'd say who gives a damn. But this is our own back yard, right next to Mexico. We have big geopolitical interests. We have to care."

In fact, U.S. attention has been intermittent, but a sense of threat close to home does wonders to focus the mind.

Central Americans, who have complicated feelings about the colossus to the north, are aware of their increased leverage.

"We don't have oil," said a shrewd civilian in El Salvador. "What we have is proximity to the United States." Given the spread of troubles around the globe, proximity imposes a priority.

A second fact, surprising to a first-time visitor, is the area's diversity. Though sharing a small isthmus and much the same early history and Spanish-Indian racial background, the four countries are strikingly different in their recent political evolution and in the problems and opportunities for current U.S. policy.

It is a bit like dealing with four adults born to the same parents in the same house, but who grew up with different life experiences, many of them violent and even murderous.

The blood-soaked histories in the area, beyond the knowledge of most North Americans, are prologue to the conflicts today. The weight of the past is accentuated by the intensely personal nature of Latin politics, especially in small countries where the main actors and even bit players know each other well, and seem unable to forgive or forget.

To all this has been added the ideological element introduced by the Cuban and Nicaraguan revolutions.

Washington always has loomed large to these small states in its shadow, and now, for several reasons, it looms larger still. The high-priority, high-profile attention of the Reagan administration has sent the expectations of governments and peoples soaring.

At the same time their economic troubles, many of which are consequences of the U.S. recession, have been deepening, with resulting political and military repercussions.

Each of the four countries I visited is in serious economic straits. The woes of the North American economy have been translated into sagging markets for their coffee, cotton and other cash exports. Moreover, the fighting and instability at home have shattered the Central American Common Market and have caused major disruptions of local economies.

The results include headlong flight of capital, rising unemployment and underemployment, growing inflation and negative economic growth. In El Salvador, Guatemala and Honduras these ills feed the fires of instability and insurgency. The Caribbean Basin Initiative recognizes this, but its private-enterprise remedies will take years to make even a dent.

Even in El Salvador, the only country where Washington is supplying large-scale economic aid, the size of the remedy is dwarfed by the size of the problem.

Expected U.S. aid disbursements this year of $140 million, plus $128 million more requested under the CBI, would make El Salvador third in the world in U.S. economic assistance, behind Israel and Egypt.

Yet this pales by comparison with a decline of $750 million to $800 million last year in El Salvador's export earnings due to sagging prices, or with the private capital flight over the past three years of $750 million to $1.2 billion, depending on whose estimate one takes.

In Nicaragua, the same economic ills, compounded by government policy, have contributed to growing discontent with Sandinista rule.

"Prices are too high, everything is controlled," said a woman slowly stirring a pot of rice over an outside fire in a countryside area. "Things are worse than in Somoza's time. We are like slaves."

More astounding, in view of the elaborate political control mechanisms, was her neighbor's emphatic interjection in a loud voice that this village "is ready for a counterrevolution." A counterrevolution would seem impossible to organize, but seasoned observers say that for the first time in a long time "more of the same" is no longer a safe forecast for the future.

The traditional U.S. answer for political strife, free elections, has been applied here recently with important though sometimes unexpected results.

Guatemala's March 7 national election led inexorably to the March 23 military coup when junior officers rejected the outcome.

El Salvador's March 28 national election gravely weakened the position of the Christian Democratic "centrists" on whom Washington had been betting, and restored the political power and authority of the right.

In both those cases the effect of elections was to precipitate political change rather than to settle outstanding issues. Only in Honduras, where U.S.-backed elections last November brought a return to civilian rule, did the ballot box work as Washington had hoped. That outcome is still tentative because of growing economic and military difficulties in the country.

Nicaragua's leaders, while promising elections of some sort by 1985, insist disdainfully that "raffles" at the polls are no way to make fundamental decisions.

For U.S. policy makers, the rapid changes in the area pose pressing questions in each of the four countries:

El Salvador

The immediate problem is that the March 28 election, while a powerful symbol of popular quest for peace and disdain for guerrilla positions, was substantively a rejection of the "centrist" Christian Democratic elements backed by the United States.

Washington's vision of victory in El Salvador had been to hold off the left militarily while building a political center to sap its popular appeal. The election, however, brought to power rightist groups which bitterly oppose the Christian Democrats and which basically disagree with buying off the peasantry through government reallocation of wealth.

To paper over this abrupt change, a moderate banker, Alvaro Magana, has been installed as interim president, and cabinet posts have been divided among rightist and centrist factions under heavy pressure from Washington and the Salvadoran military. But, in view of the fierce antipathy between the right and center, the "unity government" is likely to be unstable and uneasy.

The strong U.S. public and congressional reaction to the sudden involvement in El Salvador increasingly has limited the administration's maneuvering room. Now the political situation inside El Salvador, which is far more complicated than before March 28, has placed additional limits on U.S. postures and actions.

The chances for serious moves toward a negotiated settlement of the Salvadoran fighting seemed slim before March 28, and they appear to be minuscule today. Thus the central demand of U.S. critics appears of only rhetorical and theoretical significance.

On the battlefield, the administration is hoping for military gains based on improved training, communications, equipment and intelligence, and on new tactics, including a national amnesty program for guerrillas.

Washington also hopes through military or diplomatic means to limit the weaponry and other aid flowing to the guerrillas. It hopes recent signs of disunity among guerrilla groups presage broader splits in the future. Hopes spring eternal, but in El Salvador, so do disasters.

Guatemala

The March 23 military coup engineered by junior officers suddenly opened new possibilities in that country and for Washington in a situation which had been slowly but steadily deteriorating. What can be realized from those possibilities is unclear.

Before the coup, administration officials now concede, U.S. policy was frozen because of strong public and congressional revulsion against the bloody policies of the Guatemalan regime. The coming to power of the new junta, pledged to respect human rights while continuing the war, has provided at least the prospect of permitting substantial U.S. support.

The U.S. Embassy reports that political killings of non-combatants by government or rightist forces have fallen off almost to zero since the coup and that some 2,000 bodyguards and private soldiers are now unemployed, according to State Department officials.

These are hopeful signs for the sales job to Congress, which remains cautious and skeptical about greater involvement in a another murderous and messy war.

U.S. officials and some of the Guatemalan military would like to "sheathe" a counterinsurgency program in civic action and public works, especially for the Indian population, which is the main guerrilla target.

Such a program was tried in Guatemala at the urging of Washington in the 1960s. A major renewal of it would be costly. Congress has been told that no major new U.S. aid for Guatemala is contemplated before fiscal year 1984, which is 17 months away.

Due to economic and military woes and the expectations that have been raised, it is highly doubtful that the Guatemalan junta can wait more than a few months to assess the degree of overseas support and establish its strategy in civil and military fields.

Uncertainty about the strength of the junta leader, Gen. Efrain Rios Montt, adds to the complications. Failure of Washington to decide and act quickly could doom Rios Montt and close off some of the possibilities opened by the recent coup.

Honduras

The country in the eye of the storm, with strife-torn nations on every side, Honduras is the one that has been the least embattled over recent years. Now that is beginning to change, raising new issues and challenges all around.

The newly elected civilian president, Robert Suazo Cordova, works closely with the chief of the military, Gen. Gustavo Alvarez Martinez, and both have been helpful to the United States.

With the Honduran economy in desperate shape and internal conflict growing, they are looking to Washington for larger-scale help and, so far, have been disappointed at the response.

Aware of the greater priority given to El Salvador, senior Honduran officials as well as private leaders are asking, "Is it necessary for a civil war to break out here to get full-scale U.S. support?"

A whole series of questions is posed for the United States: to what extent should the United States use Honduran aid and territory to shut off infiltration and supplies intended for the Salvadoran guerrillas?

Should Honduras be used for a base for military pressure on Nicaragua through support for counterrevolutionary bands at their common border?

What are U.S. obligations and responsibilities in case Honduras finds itself in hot water for these or other reasons?

How much priority and resources should be given to a country where troubles are only starting to become serious?

What should be the relative balance of economic and military aid?

President Suazo is being invited to Washington for an official embrace in the near future. At that time, or before, the administration will have to make some new decisions about its plans for Honduras, which is the most dependent of the Central American countries on the United States.

Nicaragua

The Sandinista government, as suggested above, is in more trouble than at any time since the July, 1979, revolution. The trouble been brought to a head in part by the recent charges against the regime by Eden Pastora, who was probably the most popular leader of the revolution and who is now embarked on a psychological warfare campaign from Costa Rica.

As a revolutionary country with communist ties in the midst of the U.S. orbit, Nicaragua is in a complicated situation, physically, politically and psychologically. The big question is whether it will find a way to live within the limits of U.S. tolerance or, moving increasingly into the Soviet-Cuban orbit, will seek to defy those limits.

In recent weeks, Managua suddenly became eager to begin high-level negotiations with Washington about the issues between the two countries. In the U.S. view, the crucial issue is Nicaraguan aid to Salvadoran rebel forces. But as Managua became unexpectedly eager, Washington turned more cautious.

On the other side of the superpower ledger, the coordinator of the Nicaraguan junta, Daniel Ortega, was in Moscow last week to explore that relationship. It is unclear what he achieved beyond expressions of friendship.

The administration, which is pessimistic about a negotiated truce with Nicaragua, faces major decisions about the timing, procedures and substance of the proposed negotiations.

Fundamentally, it must decide whether to continue on the path of economic and political pressures and, reportedly, aid to covert paramilitary pressures, seeking to contain and squeeze Nicaragua, or whether to make a serious effort, which would imply major compromises on its part, to reach an accord. There is a sense in both capitals that a moment of decision is at hand.

The needle of any political seismograph monitoring the troubled region of Central America would have been moving rapidly and erratically in the spring of 1982. The United States, which has suddenly awakened to the rumblings, is certain to play an important role in determining the nature, scale and consequences of the next eruptions.

Despite the geographical proximity, it is clear to a visitor that the deeply ingrained political and social relationships within these countries are not susceptible to manipulation from Washington, especially in view of the disparity between the imposing problems and the relatively limited U.S. resources available for dealing with them.

Washington cannot "cure" the domestic ills of Central American nations nor, in view of domestic constraints, fight the wars of these nations for them. Probably the most effective and important thing that the United States could do in the region, as in so many others, is to restore the vitality of the U.S. economy, whose fortunes deeply affect those of lesser states.

The United States possibly can, though diplomatic and/or military means, deal with the superpower dimension, preventing or retarding external aid to revolutionary forces.

This has been the central focus of administration policy but, while a significant issue, does not seem to be the central focus of the governments in the area.

Central America, more than ever, is looking northward for answers, which are certain to be supplied by default or by decision in weeks ahead.