In his Fourth String Quartet, written between July and September, 1928, Bartók implemented his “bridge form” for the first time. The order of the movements is strictly symmetrical. The slow movement, which is the core of the work, is flanked by two scherzos, which are based on the same thematic material, and the outer ring is also formed by two allegros built upon the same themes. Beethoven’s quartet in C sharp minor, opus 131, might have been a distant model for the piece. Its seven movements form the same kind of symmetrical entity, though they do not follow an equally strict scheme.
Symmetry is, however, only the external characteristic of the bridge structure. Ernö Lendvai has pointed out that the key to its content can be found in the Cantata Profana. In this poem, a mythical moose is followed by nine youths who themselves change into moose as they cross a bridge; there is a metamorphosis in which substance remains the same while guise changes. The same also takes place in the Fourth Quartet; when the middle part, the bridge, is crossed, the material takes on a new nature and significance, but still essentially remains the same.
The first Allegro follows the classical sonata form. The melodic material is chromatic, and the skilfully polyphonic weave is dissonant, even aggressive. The first scherzo, Prestissimo, is played from start to finish with a sordino. The form is A B A. The tone, veiled by the sordino, is continually varied by changing the manner of playing. Non troppo lento also follows the A B A form. In the first section, the melody is carried by the cello; in the second, first by the violin and then by the viola; and in the third, the inverted melody is divided between the first violin and the cello. The movement ends with a brief coda. The second scherzo, Allegretto pizzicato, is played pizzicato from beginning to end. The theme in the first section of this movement, which has an A B A form, corresponds to the main theme in the second movement. Its appearance has, however, become diatonic, and its range has correspondingly expanded from a fifth to an octave. The last movement, Allegro molto, is a free variation of the first movement. It closes with a coda, the first half of which is an almost literal repetition of the end of the first movement.