Chinese four-character phrase or idiomatic expression
Refers to set phrases or idioms composed of four Chinese characters, often with classical or literary origins. Similar to 四字熟語 (よじじゅくご), but 四字成句 may emphasize the fixed, idiomatic nature of the expression.
"Shimensoka" is one of the famous four-character idiomatic expressions.
四字熟語 is the more common term for four-character idiomatic compounds, while 四字成句 is a less frequent synonym that may stress the fixed, proverbial nature of the phrase.
From 四字 (four characters) + 成句 (set phrase, idiom). The term is a straightforward compound describing a fixed expression made of four kanji.