A senior U.S. official, seemingly contradicting President Reagan's national security adviser, said yesterday that the views of Central America's four democratic presidents are unlikely to have any major effect on the upcoming congressional vote about continuing military aid to Nicaragua's contra rebels.

The official, who declined to be identified, said that the United States expects the outcome of a summit meeting in Costa Rica today between the four and Nicaraguan President Daniel Ortega to have "an ambiguous result" that will be "largely forgotten" by Feb. 3 and 4 when Congress is to decide the contra aid question.

His assessment appeared to run counter to what another senior administration official told reporters Tuesday about the visit to Central America last week of Reagan's security adviser, Lt. Gen. Colin L. Powell.

The official at the earlier briefing said Powell had told the presidents of Costa Rica, El Salvador, Honduras and Guatemala that the fate of the contra program depends, to a large degree, on how Congress perceives their attitude toward the insurgents fighting Nicaragua's Marxist Sandinista government.

According to the official, Powell argued that unless they signal Congress that they believe Nicaragua is hindering implementation of the Central America peace process and should be subjected to further military pressure from the contras, Reagan's six-year commitment to the rebels probably will be doomed.

The official's account of Powell's mission essentially was confirmed Wednesday by White House spokesman Marlin Fitzwater, who said the actions of the four presidents "will guide our funding requirements and that in turn will determine the future of the resistance."

Yesterday, however, a different senior official summoned reporters to offer a different perspective on the Costa Rica summit which will assess progress of the regional peace process that began last August. This second official insisted that he was not disagreeing with Powell because, he acknowledged, a "clear and strong statement by the presidents either for or against the contras obviously would have great impact on what Congress does."

But, this official continued, soundings by U.S. diplomats indicate that except for Costa Rican President Oscar Arias, an outspoken opponent of continued contra activity, the Central American leaders will be unwilling to make a clear-cut statement about continued contra support and thus are unlikely to have much influence on the debate in Congress.

The official said the Costa Rica meeting is expected to extend the deadline for implementing the peace plan, probably until May or June.

As a result, the official added, the administration will concentrate on tailoring its aid request in a way that might convince wavering members of Congress that it will help to advance the peace process.

That, the official continued, will involve such questions as the timing and size of aid disbursements to the contras during the extended implementation period, and appropriate types and quantities of weapons.

Under an agreement with Congress last month, Reagan must make a formal aid request later this month.

The official confirmed that the target date is Jan. 26, a day after the State of the Union address. But, the official insisted, the cost and duration of the package have not yet been decided since they depend on what happens in Costa Rica.

Last fall, Secretary of State George P. Shultz said the administration intended to ask for $270 million in new contra aid. More recently, the administration has refused to specify amounts but there has been speculation that the administration wants as much as $12 million a month in military and nonlethal aid running at least through fiscal 1988, which ends Sept. 30.