The trade bloc comprising Brazil, Argentina, Uruguay, and Paraguay is Mercosur, short for Mercado Común del Sur, or Southern Common Market. In Portuguese, it is commonly written Mercosul.
The four countries created Mercosur by signing the Treaty of Asunción on March 26, 1991. Their aim was to expand regional trade, reduce internal barriers, and coordinate commercial policy. The Protocol of Ouro Preto, signed in 1994, gave the organization its institutional framework and legal personality.
Mercosur is often described as a customs union, although its integration is less complete than that of the European Union. The founding members sought free trade within the bloc and a common external tariff, but implementation has included exceptions and political disagreements.
The question names the four founding countries, not necessarily every country associated with the bloc today. Bolivia later joined as a member, while Venezuela’s membership has been suspended. Chile, Colombia, Ecuador, Peru, Guyana, and Suriname have had associated status. Those later developments do not change the canonical answer: Mercosur.