What is "yotsugana"

are a set of four specific kana, じ, ぢ, ず, づ (in the Nihon-shiki romanization system: zi, di, zu, du), used in the Japanese writing system. They historically represented four distinct voiced morae (syllables) in the Japanese language; however, today, in standard Japanese and the dialects of most Japanese speakers these morae have merged down to two sounds.