In a setting of splendid informality, four Western leaders thrashed over some of the world's problems here today at a summit conference that the British used to announce a near agreement for the sale of Harrier jet fighters to China.

British Prime Minister James Callaghan informed President Carter, French President Valery Giscard d'Estaing and West German Chancellor Helmut Schmidt of the impending jet deal this morning as the four men chatted in an open-air, thatched-roof cabana some 30 yards from the gleaming blue Caribbean Sea.

Sources at the unusual, unstructured four-power conference on this French-ruled resort island said there were no objections to the jet sale from the other three, including Carter, the leading advocate among the leaders of a halt in the growth of conventional arms sales around the world.

Callaghan, the sources said, stressed that the jets are considered "defensive" and would be only part of an overall "balanced" trade agreements with the Chinese also involving nonmilitary items. The package could total as much as $2 billion in British sales to the Chinese this year. The number of jets involved was not disclosed.

Despite protests from the Soviet Union, Britain has been known to be negotiating the jet sale with the Chinese and Callaghan's announcement did not come as a startling surprise. But Callaghan faces national elections this year and British officials deftly used the first full day of the Guadeloupe summit to guarantee the prime minister heavy press coverage back home.

The four leaders, casually dressed and sitting around a table in the ocean-front cabana met for more than three hours this morning.

Pierre Hunt, press spokesman for the host French government, later told reporters the four men discussed general world political conditions and agree that the U.S. decision to establish diplomatic relations with China "should not be detrimental to the policy of detente" with the Soviet Union.

U.S. officials obviously were pleased with this expression of support for Carter's China decision, which was described by the other three as a "positive factor."

Hunt said they also discussed energy and relations with developing nations.

The President, officials said later, reported to the others on the status of the strategic arms limitation talks wih the Soviet Union and the prospects for a future SALT III accord dealing with tactical nuclear weapons -- of interest to the Europeans since they will affect directly Western Europe's defenses against a tactical nuclear threat.

At dinner last night, the initial gathering of the four leaders, the president provided the Europeans with the latest U.S. assessment of the situation in Iran, an important source of oil for the four countries and a subject of deep concern to their leaders.

Iran, U.S. sources said, also was discussed this morning and is expected to be the subject of a more detailed analysis by the four here Saturday, the last day of the summit.

But little, if anything, concrete is expected to emerge from these talks, described repeatedly as "informal," with no prearranged agenda or expectation of formal agreements or decisions.

This afternoon, the four leaders, this time each accompanied by one aide, met for a second time to discuss security issues in a setting that was conducive, to say the least, to an attitude far removed from weighty political matters. The many women in topless bathing suits lounging around hotel swimming pools were the subjects of as many conversations as the comings and goings of the world leaders.

Carter was up early this morning and jogged about two miles along the golf course of the resort hotel where he is staying. After breakfast with his wife, Rosalynn, he joined the other leaders for the first round of talks. At lunch, the leaders and wives were served native gumbo soup, suckling pig and hearts of palm salad at a nearby villa.

The host government flew in hundreds of police and military personnel from France to provide security, only to be confronted with an embarrassing lapse this morning. Three American reporters casually strolled into an outdoor lounge only 25 yards from the cabana where the talks were held. Sipping beer and coke, the reporters watched the entire discussion, although they could hear nothing. The French reportedly were not amused.