four strings; four-stringed instrument
Refers to any instrument with four strings, or the strings themselves. Often used in musical contexts.
A four-string guitar resembles a ukulele.
Specifically refers to the biwa, a traditional Japanese four-stringed lute. The cross-reference to 琵琶 supports this specialized meaning.
See also: 琵琶
四弦 is sometimes used as another name for the biwa.