Hello, four or five beats are mixed beats. It can be formed by the combination of four-two beats and four-three beats (four-two beats in front and four-three beats in the back) or by the combination of four-three beats and four-two beats (four-three beats in front and four-two beats in the back). The difference between these two combination methods lies in the different combination order of beats. For details, you can buy a Basic Music Theory Course to learn and understand it in detail. The following picture is the cover picture of this book!