tetrad
set or group of four entities
four go houses
traditional Japanese go schools
quartet
group of four people
quaternion
noncommutative extension of the real numbers
Four Big Pollution Diseases of Japan
group of diseases caused by pollution in Japan
Si yuan shu
Chinese quaternion invented by Zhu Shijie (Yuan dynasty mathematician) in 1303
nibble
four-bit unit of digital information
foursome
sexual act
Four Great Bodhisattvas
four Bodhisattvas in Chinese Buddhism
tetralogy
compound work that is made up of four distinct works
Solar and lunar birds
group of sculptures by Joan Miró
Dragon Kings of the Four Seas
four deities in Chinese folk religions
quatrain
type of stanza, or a complete poem, consisting of four lines
tetrad
set of four notes in music theory. When these four notes form a tertian chord they are more specifically called a seventh chord, after the diatonic interval from the root of the chord to its fourth note (in root position close voicing)
Four Cases of the Qing Dynasty
group of cases
four seasons
the four climate periods experienced in one year, on much of planet Earth