Ending a four-year tour in Peking, Arthur W. Hummel Jr., the China-born U.S. ambassador here, says he is leaving confident that "an amazing web of relationships" between the United States and China will grow stronger.

In an interview, Hummel predicted that although the Taiwan issue had the potential to be "extremely destructive" to Sino-U.S. relations, it would be manageable. He also said that for some time to come, China would face the world as a "nonthreatening" power preoccupied with its economic modernization and reforms.

"This has profound implications for our defense posture," said Hummel. "As others have said, we no longer need to plan for the possibility of a war with China."

"Our official relations are characterized by more candor and less confrontation," said Hummel, the State Department's ranking expert on China. "I think Chinese and Americans get along well, partly because we're an up-front kind of people and the Chinese can be, too, when there's a reasonable amount of trust."

"The multiplicity of relationships which we have -- perhaps the majority of them having nothing to do with the U.S. government -- is a genuine stabilizing force and a force which through the decades will produce much better understanding," he said.

The bespectacled, plain-talking ambassador said that whatever leadership changes are now in the making in Peking, he was "confident that the main lines of China's economic reforms are going to be maintained."

Based on his confidential discussions with Chinese leaders, Hummel said these main lines included China's continued opening to the West in trade, foreign investment and technology transfers; a "dilution but not abandonment of central socialist planning" that will account for market forces, and the promotion of younger leaders.

"China's new personnel system may have as much impact as anything else in the long run," said Hummel. "They are for the first time in China's long history training people, selecting them carefully for jobs, promoting them and retiring them later -- including the retirement of people who are unqualified."

Hummel said he arrived in Peking four years ago during a "downturn" in U.S.-Chinese relations over arms sales to Taiwan. After long negotiations, the two sides reached an agreement in August 1982 under which the United States would restrict the sales at current levels of quality and quantity and then reduce them gradually.

During the four years, the ambassador said, an "upturn" in relations has occurred that he thinks is going to continue.

"The combination of the very strong desire of the Chinese for modernization and their almost equally strong desire to have the United States involved in that modernization because of our leading edge in technology and managerial skills has produced an amazing web of relationships," Hummel said.

These relationships, he said, include about 14,000 Chinese students studying in the United States and about 2,000 Americans teaching and studying in China.

Hummel, who is to leave Saturday, will be replaced by Winston Lord, 47, who was president of the Council on Foreign Relations in New York.

Hummel was born in China in 1920, the son of a Congregationalist minister and noted Sinologist. He spent two years in a Japanese prison camp before escaping with the help of Nationalist Chinese guerrillas.

With a graduate degree from the University of Chicago, Hummel joined the State Department in 1950. According to an embassy colleague, Hummel, 65, is the senior American ambassador in the world, having served as U.S. envoy to Ethiopia, Burma and Pakistan.

Colleagues describe Hummel as the quintessential quiet diplomat, a problem-solver who does not often talk in terms of broad concepts.

Hummel is reluctant to talk about himself. He did say that after 35 years in the Foreign Service, he will retire after returning to Washington. He said he will do some consulting for government and for businesses and give lectures. He also plans to write a book about a Nationalist Chinese guerrilla leader with whom he fought.