SERGEI RACHMANINOFF -- that brooding, patrician musical poet of pre-revolutionary Russia -- can hardly be described as a creature of the Jazz Age. But he was one of the true lions os the musical world during that era. And once, just once, that wave of jazz influence that swept up musicians from Stravinsky to Gershwin touched this heir to the mantle of Tchakovsky.

It is his urbane, jazzy Fourth -- and last -- Piano Concerto, the first of the works he wrote after coming to the United States, and it is the single most neglected work of his mature years.

Research has uncovered no evidence that the Fourth, which came two years after Gershwin's own infectious Concerto, has ever been played in Washington. That is a circumstance that pianist Eugene Istomin and conductor Mstislav Rostropovich are setting out to rectify in tonight's National Symphony concert, and in the repeats later in the week.

The performance is not exactly a premiere, but it will be the concerto's introduction for most listeners.

Until now, the Fourth has seemed almost to carry a curse, starting with its first performance. That happened at Philadelphia's Academy of Music on March 18, 1927.

The composer was the soloist with the Philadelphia Orchestra, that then-incomparable ensemble for which he wrote all his later orchestral works. At 54, Rachmaninoff was accepted as one of the great pianists of history. He was also a fine conductor who had recently turned down the conductorship of the Boston Symphony, causing it to turn to the lesser-known Koussevitzky. And he was one of the most famous of composers. His earlier concertos--especially the mournful Second and the incredibly taxing Third (dubbed by one wag "the Macho concerto")--were ubiquitous, just as they are today.

But even the mightiest figures can take a fall. And that's what audiences and critics caused to happen to Rachmaninoff's Fourth Concerto, relegating it to derision and neglect.

Long among the dissenters from this judgment is Istomin, who has admired the Fourth ever since his youth, when he twice heard Rachmaninoff play it with the Philadelphia Orchestra, the orchestra with which Istomin's own career has been most closely linked. Still, until now Istomin has had trouble convincing conductors of the work's quality.

Thus tonight will mark the first time the pianist has been able to play the work in concert. Likewise, Rostropovich has never performed it before. A check with its publishers showed that the Fourth is programmed no more than about once a year in this country. Istomin seems to be the only American pianist of world stature who has programmed it. As he observed recently, "This will be my contribution for the cause."

It is indeed a worthy cause. The Rachmaninoff Fourth is a distinctive composition, drier and less discursive than most Rachmaninoff. The jazz references that riddle it are American, and the sound--much less expansive and voluptuous than most Rachmaninoff--sometimes is reminiscent of Ravel, whose own jazz-filled Concerto in G would come three years later.

All three movements of the Rachmaninoff concerto are full of jazz parallels. But its heart, both stylistically and emotionally, is the central slow movement, a beguiling blues, spun out in a Russian accent. It is a memorable marriage of Rachmaninoff's natural lyric gift with the new American musical language. Its five introductory bars for the piano actually sound like a jazz pianist noodling around on the keys in search of the right harmony and the tune.

Then comes a simple, melancholy blues ballad of melting beauty. It consists of the repeated lyric incantation of a descending four-note line, subjected to marvelous jazz-derived harmonic and stylistic embellishments. The orchestra picks it up and the movement proceeds. It is very much of the same character as Gershwin's blues piano prelude, which was written a year earlier--an equally hypnotic work.

Why then has this concerto been ignored? One reason is that the early critics simply did not understand it. They were expecting another outpouring of Russian angst, and when they got this concise, elliptical piece they misinterpreted it as an arid exercise, lacking in inspiration. And partly as a result of this, a frustrated Rachmaninoff kept rewriting and compressing it during the remaining 17 years of his life, making it difficult for other pianists to take it as part of their repertoires (Istomin has actually restored a few passages from earlier versions).

Furthermore, Rachmaninoff complicated things by producing in 1934 one of the impeccably perfect works that anybody has written for piano and orchestra, the Rhapsody on a Theme of Paganini. It became, and remains, the late Rachmaninoff work to perform.

The Fourth Concerto is not a creation of that quality, but neither are the other concertos, lovely as they are.

It is as good, though, as many frequently heard works. And its character is unique. Istomin is doing not only Rachmaninoff, but also today's listeners, a true favor by reviving it.