The Vienna State Opera performs from 40 to 45 different operas a year - far more than any rival - and it has become the delicate task of 'Opera director Egon Seefehiner to decide which four productions will be brought to the Kennedy Center in autumn of 1979.

Lots of factors must be weighed. There must be a proper mix; after all, it can't be all Mozart, much as some might jump with joy at that prospect. And artistic temperaments must be respected; a production conducted by Leonard Bernstein must be matched with a production closely identified with Herbert von Karajan, or Karajan might stay home in Europe.

Physical demands on the singers cannot be ignored; two of the likely productions normally feature soprano Gundula Janowitz and, since both are rigorous-parts, she will have to have some help. Also, the sheer bulk of certain sets makes transit awkward.

And hardly the least consideration is that the productions be, in Seefehiner's careful understatement, "attractive enough to merit display." That has already become the simplest part of it for the director, for he has summarily dismissed most of the Opera's repertory from even being considered - including most of the productions he inherited a year and a half ago when he took over the Opera with a mandate to give it a shot in the arm.

Seefehlner, a man of 65 who is Hitchcockian both in proportions and demeanor, minces few words when something elicits his disapproval. "Take our Wagner, for example. I know Washington doesn't get much Wagner, and I'd like to bring a Wagner opera, but I don't think I'm going to be able to line up anything that I would care to display at the Kennedy Center.

"Most of our Wagner productions need replacement, except for "Tristan' and 'Lohengrin,' and those are probably not possibilities. 'Tristan' appears to be out because Birgit Nilsson (today's leading Isolde) doesn't want to come to the United States. And I can't really come with 'Lohengrin' because I have already brought a 'Lohengrin' to Washington, when I was with the Berlin Opera in 1975.

"We are going to have to get our Wagner in shape." He shrugs and with that dismisses the present production norm at one of the world's most-noted centers for the performance of Wagner.

Among the productions that do meet the Seefehlner test are works that not only are of the highest quality but also have the added appeal of being inextricably tied to Vienna's incomparable musical heritage. Two of them, Beethoven's "Fidelio" and Mozart's "The Marriage of Figaro," were premiered in the city. And two more, Strauss' "Der Rosenkavalier" and "Ariadne auf Naxos," are (along with Johann Strauss "Die Fledermaus") perhaps the grandest musical dramaticizations of life in Imperial Vienna.

Two of these, "Fidelio" and "Ariadne," already were booked for the appearances at the Center that had been planned for next September, but were aborted late last summer by an Austrian government austerity drive that cut deeply into the budget of the largely state-supported opera.

"I think both of these are still solo for a year later. Leonard Bounstein (who will conduct "Fidelio") and Karl Bohm (the conductor of "Ariadro") are both adjusting their subsdvice. Casting the singers shouldn't give us too many problems.

"Also safe are the concert performances in the Concert Hall, where we would include some choral works like the Mozart Requiste."

The biggest question mark now concerning the 1979 visit to whether von Karajan will be along. This fall he had been committed to conduct a concert version of Verdis "Don Carlo" with on all-star cast. Seefehlner gives the impression that cancelling the unsurged "Don Carlo" would trouble him little. "Karajan really wanted to do it, and yet our production isn't one we would care to show off, so we settled for this."

Seefehlner gets much of the credit, though he does not claim it; for luring the: conductor back to the Vienna Opera last spring after a decades of self exile that followed Karajan's years as head of the Opera. Seefehlner particularly admires the new "Marriage of Figaro" staged for Karajan and would like to bring it if Karajan can make the trip. "This is one production we've done that everybody liked," he says with satisfaction.

Final choices and castings will be determined when Martin Feinstein, the Kennedy Center's executive director for the performing arts, travels to Vienna in late winter to sign the contracts.

Other possibilities mentioned by Feinstein are "Rosenkavalier" and Mozart's "The Abduction From the Seraglio."

The Center and the Opera are still debating whether the visit should come in September or March. The company will be visiting the United States for the first time and will perform in Washington only.

The $1 million that the Austrian government came up with for travel expenses falls short of the full cost of the trip, to which the Austrians were originally committed. The Center will finance the remainder, which Feinstein estimates at $350,000, plus the costs of production once the company arrives, as agreed from the beginning.

Feinstein gets credit for the idea that finally produced most of the Austrian money. If the Opera could not finance the trip, why could not the Austrian tourist agency, as an investment in attracting visitors to what Seefehlner, a Vienne native, proclaims as "the most important nation of music?"

But release of the funds, in early December, came as a surprise to Seefehlner, and later to Feinstein as well. "By then nobody had though we would go," says Seefehlner ruefully, "so it was wonderful to hear. But in a way the timing was a pity. If they had told me as little as a week before, I think we could have still made it for 1978. Bernstein had held open his dates until Dec. 1. It was tha close."

One American source in Vienna has suggested that the government may deliberately have held out the money until it had to be spent a year later, and was thus less in conflict with the austerity program. "If so," said the observer, "it would be yet another example of a classic political solution known here as "the Viennese compromise."