Why are the four ancient poems not four ancient poems?

It means different things. Poetry is a literary genre with a certain rhythm and rhythm, which expresses the author's rich emotions in highly concise language and reflects social life in a concentrated way. Ancient poetry is a poetic genre based on classical Chinese. Compared with modern poetry, its format and rhythm are limited, but it is more subtle in emotional expression, paying attention to "gentleness and gentleness, mourning without complaining", and even developing a unique traditional lyric way in China, which is intended to be beyond words and poetry. Chinese ancient poetry, commonly known as ancient poetry, refers to poetry written in classical Chinese and traditional metrical patterns, which is a generalized ancient poetry in China and can include all kinds of ancient Chinese rhymes. So the meanings expressed by the two are different.