More than 4,000 letters a day arrive in Washington for President-elect Ronald Reagan. Nearly all go directly into the hands of the West Springfield High School Republican Club.

The 40 teen-age members, who stuffed envelopes during Reagan's campaign, constitute the main body of workers who since election day have been opening the mail at Reagan's transition headquarters at 1726 M St NW.

The students carpool into town several days a week after school and on weekends to sit on the floor or on cushy piles of mailbags, where they read and sort. Once opened, the letters go into one of several dozen "issue" baskets, on topics such as ERA, gun control, the hostages, the economy. Other baskets are filled with congratulations, tips from grade school students and former government officials on how to run the government and, of course, thousands of resumes.

A lot of the mail, say the teen-age volunteers, is from children. "Reagan gets tons of kid mail, you know from kids around the country," said Leslie Clason, 15, who was holding two letters, one "from a kid who just wanted to say he was glad Reagan was president."

One letter from an adult, which is typical of many, according to Clason, was from a man who said "he was available for a $50,000-a-year job in Washington" and was enclosing his resume.

Clason doesn't much fit the mold of a dedicated Reaganite. She confessed that her parents voted for Anderson and she probably would have too, "but I've been converted by my friends and I come down here because I want to do something for the country."

Some other club members say they come from Democratic families. Carl Wescott, a 17-year-old senior at West Springfield, said, "I'm the only Republican in my family." Chris Despard, also 17, said, "My parents are Democrats but that's just so they can vote in the primaries. They're really independents. But they voted for Reagan."

Westcott and Despard said they joined the Republican club because they enjoyed its activities. "Most of us helped stuff envelopes for Reagan before the election," said Wescott. "And we also helped Parris" (Stanford E. Parris, the Republican who won Northern Virginia's 8th District).

The Young Republican Club was organized at West Springfield High four years ago, during the 1976 presidential campaign. Mike Bullock, a 37-year-old geometry teacher who supported Gerald Ford in 1976, helped start the club.

"We had kids down here in 1976 stuffing envelopes a block away at the Ford headquarters on 18th Street," said Bullock. "But the real reason for the club is to involve the kids in the governmental process. Not only do we help here, we usually go on trips at the end of the day, to the Washington Monument, on an underground tour of the Lincoln Memorial, to the National Archives and Ford's Theater. I'm from Kansas and a lot of the kids are from out of state and haven't seen these things."

The Republican club is one of the most active at West Springfield and the only successful political club, Bullock said. "Other teachers started a Democratic club for Carter last spring. But it fizzled out after a few months."

Regardless of political affiliation, Bullock said, the club offers a great opportunity for teen-agers to learn about government.

"They feel a personal involvement, too," he said, "and have met and shaken hands twice with Reagan and Mrs. Reagan. . .

"And what a way this is to learn history. (Alexander) Haig was in here one day and the next they all read about him in the papers."

After the inauguration, Reagan's letters will be processed by the White House staff. President Carter, for instance, received about 40,000 letters a week before the election, most on specific issues, according to a White House spokesman.

But until Jan. 20, the Fairfax County high school students, nourished on hamburgers from a nearby fast food restaurant, will continue their jobs of sorting things out for Ronald Reagan.