American Music Week came a bit early for the National Symphony, which will include William Schuman on its program next week but had programmed Beethoven and two French composers for last night. Fortunately, there is plenty of American music available on short notice. So last night's concert, conducted by Rafael Fru hbeck de Burgos, opened with an unexpected world premiere that took only slightly longer to play than it does to describe.

Leonardo Balada's "Wedding Dance" is part of a work in progress, the opera "Zapata," which is scheduled for its first performance in Pittsburgh in March 1988 with Sherrill Milnes as Mexican Revolutionary Emiliano Zapata. If the dance is a fair sample, the opera will be a rhythmically lively, melodically busy and harmonically sweet-and-sour experience, with a strong flavor of Mexican folk music and lots of percussion to enliven its wild, rather disjointed proceedings.

Whatever the rest of "Zapata" may be like, the dance opened last night's concert like a spoonful of tabasco sauce at breakfast. It sounded underrehearsed, but I think it is supposed to sound that way. And it made the National Symphony, briefly, the world's largest mariachi band.

Jeffrey Kahane was the soloist in the melodious and intricately brilliant Piano Concerto No. 4 of Camille Saint-Sae ns, and he did the music complete justice. It is good to see the orchestra recognizing the 150th anniversary of this fine, still perhaps underrated artist, who was born while Chopin, Liszt and Mendelssohn were still young men and continued his creative life well into the 20th century. During more than 60 active years as a composer, Saint-Sae ns produced abundant material for anniversary performances, and we may hope to hear more of it before his 151st birthday next October.

Ibert's "Escales" ("Ports of Call") is an orchestral showpiece that seldom appears on orchestral programs any more, as Richard Freed justly laments in this week's program notes. It has fared somewhat better on records and seems to get a new lease on life with each advance in recording technology; so perhaps we will hear more of it in the dawn of the digital era. At any rate, this Mediterranean travelogue gave the orchestra a chance to stretch its muscles last night, particularly in the languorous Arabic second movement and the vivid Spanish finale. Oboist Rudolph Vrbsky rightly got a special round of applause for his work in this music.

The most substantial work on the program was also the best known: Beethoven's lyric, shapely Fourth Symphony. Fru hbeck's interpretation featured finely calibrated dynamics, excellently coordinated ensemble sound, a high level of carefully controlled energy and an acute sense of the music's shifting moods. It was beautifully performed.