When did ancient poetry change from four words to five words?

In the history of China literature, the earliest poem we can see is The Book of Songs, which is about 3000 years ago. This is the earliest collection of poems in China, with a total of 305 poems, which was called "Poetry" or "Poetry 300" in ancient times. Its content consists of three parts: wind, elegance and praise, which are divided from the perspective of music. There are three ways of expression: Fu, Bi and Xing. Therefore, predecessors called it "wind, elegance, praise" and "fu, comparison, xing" six-meaning poems. Syntactically, language is basically four words and one sentence.

Following the Book of Songs, in the 4th century BC, a new style of poetry appeared in the State of Chu, called Songs of the Chu, and its founder was Qu Yuan. Later, the Han people compiled the works written by Qu Yuan, Song Yu and others into a book called Songs of the South. The Songs of Chu broke through four sentences in the Book of Songs and developed into five-character sentences and seven-character sentences, that is, even sentences (four words in a sentence) were changed into odd sentences (five words in a sentence and seven words in a sentence), which not only better expressed thoughts and feelings, but also made the rhythm and rhythm more musical.

In the Han Dynasty, Yuefu Poetry, a poem sung with music, appeared. There are four words, five words and miscellaneous words in language, but most of them are five words. Later, the literati headed by Cao Cao and his son and Tao Yuanming developed five-character poems. At the same time, seven-character poems have also developed greatly.