Bartók's penultimate quartet was composed in 1934 on a commission from Elizabeth Sprague Coolidge, at the time America's most prominent musical patron. It represents a mellowing of Bartók's quartet idiom; the sounds are less dissonant and less harsh than in his previous quartets.
As he often did, Bartók uses an "arch" form, with a central scherzo sandwiched with two slow movements, which are in turn bookended by energetic fast movements. In the opening movement, with its banging rhythmic motifs and sinewy melodic ones, this mirror idea is carried out on a smaller level, with the melodic material at the end of the movement consisting in large part of earlier themes in mirror image (i.e. played backwards or upside down). Such techniques are more apparent on paper than to the listener.
After a delicate second movement, with its short, halting phrases over soft but rich harmonies, comes a Scherzo marked "in the Bulgarian style." Like Bulgarian folk music, it features strong but uneven rhythms. The meter is notated 4+2+3 /8 (worlds away from a simple 9/8) and changes to an even more complex 10-beat pattern in mid-movement.
The fourth movement, like the second, begins tentatively and features affecting harmonies, most strikingly toward the end, when the violins sound celestial chords over short runs in the viola.
The finale has a furious urgency, but there are also moments where it gets silly or sassy. Toward the end, where we would expect a climax, the action stops for what sounds like a mildly out-of-whack carousel: the lower parts take up a frivolous tune in A (actually a variant of a theme already heard in the movement) and the first violin comes in with the same tune in B-flat, oblivious to the clash. The players are directed to play "with indifference."
— Howard Posner plays lute and Baroque guitar and practices appellate law in Los Angeles.