Federal officials said yesterday that existing flights at Washington's National Airport will be cut back about 4 percent on Dec. 1, a less severe cutback than they estimated earlier this week.

Federal Aviation Administration Chief Counsel J. E. (Sandy) Murdock III acknowledged yesterday that the 4 percent reduction in flights for December, on top of the already curtailed flight schedules, would mean that holiday travelers would find at least 20 percent fewer flights out of National this Christmas travel season than last.

FAA officials said that numbers they gave The Washington Post earlier this week about proposed December cutbacks--suggesting a severe 21 percent cutback from current levels at National--were in error. "It was my fault," FAA Chief spokesman Chuck Murchison said, adding that the cutback figures given out really referred to cutbacks from prestrike levels, rather than from existing levels. Flights at National currently are off between 17 percent and 18 percent, he said.

Additional flight cutbacks at National and 19 other airports where flights already are curtailed are part of an FAA plan to try to reduce flight delays and give the understaffed air traffic control system some added flexibility for dealing with adverse winter weather.

The FAA wants to reduce airline activity nationwide to below 78 percent of the operating level before the strike Aug. 3 by the Professional Air Traffic Controllers Organization. Although flight reductions imposed at 22 major airports initially reduced the system nationwide to 75 percent of the prestrike level, the FAA routinely has allowed airlines to add flights, and airline activity now is about 85 percent of prestrike levels.

The flight cutbacks ordered for Dec. 1 vary considerably at the 20 affected airports, but are most severe in areas where the walkout was most disruptive: the Northeast and the Midwest. Murdoch said yesterday that flights could be reduced by 10 percent from present levels at New York's LaGuardia Airport and 9 percent at Chicago's O'Hare Airport. He emphasized that all the figures were tentative and could change. Some of the other airports will have to cut flights during just one or two hours. Two of the original 22 restricted airports--Miami and Kansas City--will suffer no additional reductions.

Although officials of some airlines privately have expressed concern about the method the FAA may be using to allocate flights among the airlines, Murdoch insisted yesterday that the FAA's process was totally mechanical and that it was not choosing one airline over another. "We don't look at the markets being served," he said. "We're only looking at the hours served and the capacity the system can handle."

Despite his assurances, some airline officials have been complaining that the FAA is making decisions that could affect their economic viability in total "confusion," and that they don't know the basis for the decisions.

"You don't know who's regulating you; you don't know what the rules are . . . and you don't know what the remedies are," said Michael L. Muse, president of Muse Air, a new airline that began service this year in Texas. He said the FAA has disapproved his airline's proposal to institute some new services.