Because it is Chinese-style music with five tones and twelve temperaments, fourths are rarely used because it is too modern. The seventh can be used to modulate the key.

From the perspective of composition, if there is a lack of 4, and 7 in the composition, and Do (1), Re (2), Mi (3), So (5), La (6) are used as Lord, then you can be sure that a song is based on five tones and twelve rhythms, which is Chinese style or ancient style music.

The characteristic of Chinese style is that the chorus part is based on the ancient Chinese five-note twelve temperament and does not use the Western twelve equal temperament. Some of the melody modulations of the main song use the minor second semitone 7, which is the lower level of 1.

The intuitive expression on the simplified musical notation is: Chinese style music uses, do (1), re (2), MI (3), So (5), La (6) are the main five tones. Compared with the Western twelve equal laws, fa (4) and xi (7) are missing. For example, a very simple Chinese style music - "Blue and White Porcelain" by Jay Chou. The entire melody score of Blue and White Porcelain uses 1, 2, 3, 5, and 6 as fixed anchor points, matching the interval relationship of upper and lower octaves. Such five-tone songs are actually the pitch of Zhengyu, the Gongshang Jiao in ancient China, which tends to leave a blank space for the ancient style and charm.

If in the melody of the song, in addition to 1, 2, 3, 5, and 6, there is also a xi (7), then generally speaking, this is a chorus with a Chinese style/ Ancient style, music using xi (7) in the main song or C section modulation. Broadly speaking, this is the most common type of composition in Chinese style or ancient style circles.

So why is this happening? Because from the perspective of musical intervals, Fa (4) represents the fourth interval. The fourth interval has the highest harmonious status in composition. It is the interval with the most sense of justice and sonority. Simply put, it is the fourth tone represented by Fa (4). It was only included in the common tone in the West in the Middle Ages and has a strong sense of modernity. It does not belong to our Chinese five-tone twelve-tone system.

When composing ancient-style compositions, fourths must be avoided. In the chorus part, fourths and sevenths must be avoided, because sevenths are essentially dissonant intervals and have minor key characteristics. The ancient musical intervals are essentially a combination of the most harmonious five intervals.