When you say "flat format", it is called "meter". Poems written according to metrical rules are called metrical poems, which only include seven laws and five laws, as well as seven laws and five laws written with their semi-metrical rules. In order to distinguish it from ancient poetry, it is called "modern poetry".
On the contrary, four-character poems are not only modern poems, but also the most "ancient" poems in ancient poems. It doesn't have rhyme or rhyme, and it doesn't pay attention to even words (only cadence is enough). Sentences are not necessarily four words, and different words can be mixed.
So, is a four-character phrase a four-character poem? Of course not.
Because the achievements of four-character poems are concentrated in the Book of Songs, we can try to figure out something by reading the Book of Songs more. Most of the four-character poets after the Book of Songs are looking for feelings in the Book of Songs.
For example:
1, basically rhymes. There is no rhyme book in the era of ancient poetry, but it rhymes roughly. Rhyme feet can be flat, wrinkled or changed in the middle.
2. The sentence pattern is symmetrical. For example, "an old horse lurks, aiming at a thousand miles" and "a martyr is in high spirits in his twilight years" seem to be opposite at first glance, but they are not. Because their sentence patterns are symmetrical.
3. Chapter cycle. The "chapter" we are talking about today is a passage in a poem. Four-character poems usually have obvious similarities at the beginning of chapters, and the sentences in each chapter are very similar. For example:
I ride a gorgeous stallion, and the six reins in my hand are flexible and light. ……
I rode a magnificent black horse, and the six reins were as smooth as silk. ……
I'm Marlowe, and I'm in a state of chaos. ……
I'm Ma Weizhen, company six. ……