Rhodesia's interim government announced today that a coalition government will be established next April that will guarantee the minority whites a powerful and entrenched role in the government for five years.

The plan, endorsed by the interim government's black as well as white ministers, would effectively water down black majority rule until at least 1984. It provides that each party be represented in the Cabinet on the basis of its representation in Parliament.

Whites, who make up only 4 percent of the country's 6.8 million inhabitants, already have been guaranteed 28 seats in the 100-member Parliament under an agreement last March that set up the biracial interim government to organize elections for majority rule.

The March agreement said nothing about a requirement that the Cabinet reflect the proportion of parties in Parliament. It was assumed that whatever party or coalition of parties gained a majority would form the government of its choosing, as is the customary practice in British-style parliamentary governments such as that envisioned for Rhodesia.

By in effect ruling out an all-black government, the arrangement announced today seems to limit the impact of the elections before they are held.

In any case, there is a growing feeling that the elections may become impossible because of the escalating war between the government and Patriotic Front guerrillas. The elections, originally scheduled for December, already have been postponed until April.

Washington Post correspondent David B. Ottaway reported from Lusaka, Zambia, that the arrangement announced today by Prime Minister Ian Smith could set the pattern for establishment of a permament government without the elections. Based on the coalition system described by Smith, this would give the whites an important role in the government even if the election were never held.

Under the system set up by the interim government's executive council, whites would be assured of five or six Cabinet positions, giving them sweeping influence over decision-making. This was explained by Smith and his black colleagues in the interim government by the need to ensure security, political stability and economic revival.

Their statement said, "The need for a period of political stability is fully appreciated and ministers... have agreed unanimously that this would best be achieved by establishing a government of national unity in which party considerations would be secondary to the national interest and in which all parties commanding a reasonable degree of support from the electorate would participate."

Bishop Abel Muzorewa, whose United African National Council was expected to win by far the largest number of votes in any election, would appear to have given up the most under the new arrangement.His party alone might well have won three quarters of the black seats in the new Parliament and could perhaps have shut the whites out of the government.

Smith said that while Muzorewa and his followers believed that they could form a government, they had decided to accept the move "in the national interest."

The Rev. Ndabaningi Sithole, another member of the biracial government who has publicly asked Smith to stay on in some position of power after independence, commented: "We do not want independence and then a coup and then after a week a counter-coup, and then a counter-counter-coup."

A further agreement announced after today's joint session of the Executive Council and the biracial ministerial council was the decision to adopt the proportional representation system for black voters. The country is to be divided into eight provinces based on existing administrative boundaries to which parliamentary seats will be allocated in proportion to the numbers of potential voters in each region.

Voting in the provinces will follow a party list system, on the basis of symbols representing the three black organizations in the March agreement. The 72 black seats in Parliament will then be allocated according to the proportion of votes gained by the respective parties.

Although the March agreement established a common voter roll for black and white, the 28 white seats are to be elected on a preferential basis that allows whites to vote exclusively for their brethren.

A further decision announced today was a plan to allow regional authorities to have direct control over such matters as health and education.

This partially meets demands by white right-wingers for a federal system of government that would allow them to protect their ethnic interests. One such regional authority would undoubtedly fall under white control and would thus allow for continued segregation of health and educational facilities.

The new arrangements by the interim government were one reason cited by constitutional expert Ahrn Palley for his resignation today as Muzorewa's adviser. Palley, a respected but highly critical member of the Rhodesian Parliament, said: "This will lead to a Walter Mitty constitution. It can only strengthen the external forces of the Patriotic Front. To pretend that such policies can lead to a peaceful solution in Rhodesia is ludicrous."