Since May 18 the much vaunted tranquillity of this central African nation has taken on an air of menace after the deaths of three Cabinet ministers and a legislator in circumstances that have raised questions the Government will not answer.

Dick Matenje, Aaron Gadama, John Sangala and David Chiwanga were driving in the south of the country when, by official accounts, their car went off the road and plunged down a 100-foot embankment.

The official version goes no further; it does not explain why, according to a highly placed Western source, the bodies bore the marks of gunshot wounds. Neither is an explanation offered for the absence of a state funeral, or official condolences to the relatives of the dead - courtesies that would have been usual after a car accident. In the byzantine world of Malawian politics it is unlikely the mystery will ever be publicly unraveled.

The deaths have worried Malawians and foreigners alike for they seem to indicate the hardening of an already tough regime that brooks no opposition or suggestion of dissent. Landlocked and Poor

Malawi, landlocked and poor, has been led since its independence from Britain in 1964 by Dr. Hastings Kamuzu Banda, the octogenarian life President, elder of the Church of Scotland and idiosyncratic figure whose control of events is believed by some foreigners to be slipping. The official motto by which he has exercised power is known as ”the four cornerstones” – unity, loyalty, obedience, discipline. Among the objects on sale in Malawi’s well-stocked stores are wraparound cloths bearing the President’s portrait above the ”four cornerstones.”

Since coming to power, President Banda has run the nation as a fiefdom. He is the biggest stockholder in the country’s biggest conglomerate and the law has been amended to insure that his supremacy cannot be challenged. The country has a one-party system under which the life president selects those who will be favored with office and deals summarily with those perceived as threatening.

Yet in the past the methods used to maintain control have usually been coated with a veneer of legality. Suspected challengers, like Aleke Banda, the former secretary general of the Malawi Congress Party, the country’s sole legal political body, are in detention. Traditional courts have handed down death sentences to notable opponents, like former Justice Minister Orton Chirwa and his wife, Vera, but they may still lodge appeals.

The events of May 18 have woven more sinister strands into the national fabric. ”Either the President condoned what happened,” a foreign specialist said, ”or he was unable to prevent it.” Like most people here, the specialist agreed to discuss events only in return for anonymity. Succession a Touchy Topic

The prevalent interpretation of the incident is that the four officials who died on May 18 were somehow embroiled in a factional battle over the highly sensitive issue of who will succeed Dr. Banda. This is probably the most contentious and potentially hazardous topic in the land, and one that seems to foreshadow bitter political strife after his death.

While it is not a legal offense to discuss the succession, Malawi’s recent history has shown that those mentioned as likely contenders for the presidency have been politically neutralized.

The May 18 killings, the foreign specialist said, may have been the result of ”overzealous” police action ordered to prevent the four officials from seeking refuge after losing the power struggle. Mr. Matenje had been summoned before the President and admonished prior to his death, the specialist said. ”They knew they were being pursued,” the specialist said, ”but the killing was crude and messy.”

The clues as to who may have ordered the police action are diffuse. Several days after the officials’ death, the managing editor of the Government-controlled Daily Times was called to the presidential palace, a Western source said, and, one day later, an editorial appeared that was widely interpreted as an exoneration of the highly influential central bank governor, John Tembo. ‘Official Hostess’ to President

Mr. Tembo is the uncle of Cecilia Kadzamira, whose title is ”official hostess” to the bachelor President. The relationship, by most accounts, has Machiavellian overtones.

”Tembo and Kadzamira control what the President sees,” the foreign specialist said. ”Matenje may have been trying to keep things from the President himself so as to strengthen his position against Tembo, but lost the battle.”

The President, the specialist said, no longer devotes himself fulltime to the running of the country, has ”lost control of the economy” and is increasingly isolated within the palace guard around him. Most of those who have talked with Dr. Banda recently assert that he is in good health, but one Western visitor reported to friends that the Malawian leader did not seem to be as forceful as in the past.

”It is possible to run this country as a one-man show,” a Western source said, ”but only if the life President devotes himself to the job seven days a week. The feeling is that he is no longer doing that, so there has been slippage.”

There is little doubt President Banda has made achievements that are rare in Africa. At independence the country ranked as one of Africa’s least-developed, bereft of mineral resources. ”We have only the land and the people, nothing else,” a senior Malawian economist said. This year, for the first time in several years, Malawi will export corn to Zambia, Tanzania and Mozambique, whose socialist leaders have traditionally scorned Dr. Banda and his personal style. Dissatisfaction Is Increasing

But, increasingly, the President’s style is the object of dissatisfaction among Malawians. ”People do not like being treated as children in the paternalistic manner of his excellency the life President,” a civil servant said in a private conversation. And some of Dr. Banda’s policies seem to be rebounding.

Malawi is staunchly pro-Western, is the only African country to have full diplomatic relations with South Africa, a tie that has brought economic advantage along with political isolation from the continent’s mainstream. ”The South Africans are condescending towards us,” the civil servant said. ”They treat us as if we were another Bantustan, but we are an independent African country.”

Of the foreign journalists who received rare permission to cover last month’s parliamentary elections here, none were South African. The authorities, one source said, turned down an application from South Africa’s Argus Group of newspapers because they were displeased with the ”high handedness” of South African journalists in the past.

The elections were stictly controlled by Dr. Banda, who examined the candidates to insure a loyal Parliament. The assembly itself, a Malawian party worker said, has little power. ”This,” the worker said, showing journalists the new Congress Party headquarters, ”is the No. 1 Parliament. The other one is No. 2.”