tetrachord

A series of four diatonic tones encompassing the interval of a perfect fourth.

In music: An instrument with four strings.

The interval of a perfect fourth.

A diatonic series of four tones, the first and last of which are separated by a perfect fourth.

A scale series of four sounds, of which the extremes, or first and last, constituted a fourth. These extremes were immutable; the two middle sounds were changeable.

In music a tetrachord is any set of four different pitch classes.