4 is the fourth of six chamber music works in the string quartet medium by the American composer Milton Babbitt.
It is in a single movement, divided into twelve sections marked by (amongst other things) five different metronomic tempos, distributed 1–2–3–1–4–5–1–2–5–3–4–1.
In contrast to its predecessor, the Fourth Quartet makes extensive use of coloristic instrumental effects, such as col legno, sul ponticello, and pizzicato glissando.
[1] The basic pitch structure of the Fourth Quartet is an array containing 77 statements of the aggregate.
[2] It is the first of Babbitt's works to employ weighted aggregates.