The Fourth Brazilian Republic, also known as the "Populist Republic" or as the "Republic of 46", is the period of Brazilian history between 1946 and 1964. It was marked by political instability and the military's pressure on civilian politicians which ended with the 1964 Brazilian coup d'état and the establishment of the Brazilian military dictatorship.

This period was marked by often tumultuous presidencies of Eurico Gaspar Dutra, Getúlio Vargas, Café Filho, Juscelino Kubitschek, Jânio Quadros and João Goulart. In 1945, president Getúlio Vargas was deposed by a bloodless military coup, but his influence in Brazilian politics remained until the end of the Fourth Republic. During this period, three parties dominated national politics. Two of them were pro-Vargas — the Brazilian Labour Party (Partido Trabalhista Brasileiro, PTB) to the left and the Social Democratic Party (Partido Social Democrático, PSD) in the center — and another anti-Vargas party, the rightist National Democratic Union (União Democrática Nacional, UDN).

As the Second World War ended with Brazil participating on the Allied side, then president Getúlio Vargas moved to liberalize his own influenced Estado Novo regime. Vargas decreed an amnesty to political prisoners, including the chief of the Communist Party, Luís Carlos Prestes.

He also introduced an electoral law and allowed political parties to campaign. Three political parties introduced themselves into the national political scene. The liberal and rightist parties of the opposition against Vargas created the National Democratic Union. The bureaucrats and supporters of the Estado Novo grouped in the Brazilian Social Democratic Party. Vargas also created the Brazilian Labour Party, to the left, to group the workers' and the laborers' unions. The Brazilian Communist Party, weakened during the dictatorship, was also legalised.

The Estado Novo ended when two of his most rightist supporters, the Minister of War Eurico Gaspar Dutra, and Army Chief-of-Staff Pedro Aurélio de Góis Monteiro led a military coup on 29 October 1945. The president of the Supreme Federal Court, José Linhares, was inaugurated as president of Brazil. Linhares guaranteed free and regular elections and Vargas was forced into retirement. General Eurico Gaspar Dutra was elected president.

On 18 September 1946, the fifth constitution of Brazil was adopted, marking the country's return to democratic rule. That same year, the government created the Social Service of Industry (SESI) and Social Service of Commerce (SESC), and the General Staff, the future General Staff of the Armed Forces (EMFA).

In 1946, Dutra ordered the closing of casinos and prohibited "gambling" in the country. In 1947, he appointed Osvaldo Aranha as representative of Brazil to the United Nations, outlawed the Brazilian Communist Party, ended diplomatic relations with the Soviet Union and organized the Inter-American Conference of Peacekeeping and Security of the Continent in Petrópolis, which was attended by U.S. president Harry Truman. In October 1948, his government set up the Superior School of War (ESG), with American support. Closer relations with the United States were displayed by the formation of the Joint Commission of Brazil-United States, known as Abbink Mission, headed by John Abbink and Minister Octavio Gouveia de Bouillon.

The development strategy of the government included the "Salte Plan", which put emphasis on health, food, transportation and energy. Proposed in 1947, the plan aimed at better management of public spending and investment in key sectors in the country but only began to receive funding from the budget in 1949, being forgotten in 1951. During this period, measurements of the country's economic growth by calculating the Gross Domestic Product were first regularly published. The average annual growth of the Brazilian economy during the Dutra administration was 7.6%.