Foreign ministers of four prosperous American countries -- Canada, Mexico, the United States and Venezuela -- agreed here today to begin "immediate consultations" with governments throughout Central America and the Caribbean as the first step in a plan to cope with the severe economic problems that underlie spreading social and political turbulence in that region.

William E. Brock, the special U.S. trade representative who was part of the U.S. delegation here, told reporters those consultations would probably start within a month. Talks would also begin with other prosperous countries outside the region and with international financial institutions to draw them into an even broader coordinated economic assistance effort.

A joint communique issued at the close of a five-hour meeting and lucheon said that although many countries inside and out of the region are providing economic help, the four big donors agreed that "more comprehensive efforts" are needed based on "a consultative process" in which both donors and recipients come to a realistic understanding of what is needed and what is financially possible.

To ease fears that the new initiative might be used as a cloak for military aid or as a U.S. effort to isolate Cuba or other leftist governments such as Nicaragua and Grenada, the communique also said the ministers "concluded that there are significant advantages to an economic and social development approach without military considerations or political pre-conditions."

Furthermore, it said that "donor countries must be free to choose the countries with which they cooperate and the ways they can best be of help."

This is significant as a way around foreign policy differences between the United States and Mexico, in particular, since Mexico maintains good relations with and provides aid to Cuba and other leftist governments in the region. The Mexicans believe the Reagan administration has made too much of the Cuban and Soviet threat to the area.

Asked by reporters if Cuba could receive aid under the new initiative, U.S. Secretary of State Alexander M. Haig Jr., who headed the U.S. delegation here, did not answer directly. Rather, he explained that while there is no automatic exclusion of any country from aid, there is also no automatic obligation of a donor to include a certain country. Each country would make a decision on its own, Haig said, an arrangement that allows the United States and Mexico to continue their separate approaches to Cuba while hopefully coordinating other efforts.

Haig and the other ministers stressed that the plan was still in the exploratory stage, was only an agreement in principle at this point, and that no specific projects could be detailed until discussions with potential recipients are completed.

Despite the absence of hard new monetary commitments and considerable skepticism among many critics, Canadian External Affairs Minister Mark MacGuigan told reporters he had no trouble endorsing what has been done thus far as a "positive step forward." Haig also cautioned against too much cynicism, saying the fact of the meeting--the first of its kind at a "high political level" -- was in itself a good sign.

The ministers agreed to meet again before the end of this year. Mexico and Venezuela were represented by Foreign Ministers Jorge Castaneda and Jose Alberto Zambrano.

Although development plans for the Caribbean basin have been tried before without much success, the start on this one represents at least a preliminary political victory for the Reagan administration, which called for the meeting.

The Republican platform called for more attention to the region. European and other normally friendly governments have criticized the administration for its early emphasis on military confrontation in El Salvador and the tough anti-Cuban and Soviet rhetoric that tended to cast the region as an East-West battlefield.

Without backing away from that anticommunist theme, the new administration want to show that it understands economic and social factors underly much of the political turmoil that produces both revolution and illegal aliens fleeing to the United States -- another thing the administration wants to stop.

Throughout the trip, U.S. officials stressed that this was not to be a Washington-dominated effort.

"We don't want a mini Marshall Plan," Haig told reporters, referring to the massive U.S. solo effort after World War II to rebuild Western Europe. "We don't want a made-in-America stamp on this," he said. "Nobody's going to make a big commencement address at Harvard telling everybody how to develop," added another senior State Department official

The four countries meeting here already are the largest Western aid contributors to the region. The United States provides about $350 million annually in economic aid. Mexico and Venezuela, both oil producers, each also provide about $350 million, primarily through oil credits. Canada provides more than $50 million in the Caribbean.

As explained by State Department officials here, the United States plans no major immediate increase in outright economic aid. Rather, Washington will try to provide a more positive investment environment for private business and international trade in the region. This involves possible new bilateral or international codes or insurances that would lessen fears of losses.

The United States already imports about two-thirds of the region's goods. But officials say the administration is considering reducing tariffs still further, even though almost 90 percent of the dollar volume of these goods already comes in duty-free.

Asked if this would cause problems in Congress or with trade unions, Brock said no decisions have been made yet but that "everything will have its price."

U.S. officials also suggested a proposal may be made to create an international economic assistance mechanism in Central America similar to one already organized in the Caribbean under World Bank auspices.