What grade is the zebra reading s6?

I learned zebra reading s6, which is equivalent to grade three or four. Official website, a zebra, said that zebra reading s6 mainly explores the types of ancient poems, deep reading, bad habits and sentences and their revision methods, so learning zebra reading s6 is equivalent to grade three or four. Zebra reading is divided into s 1-s6, which cultivates reading core literacy such as listening and speaking, reading and writing, Chinese character cognition, literature appreciation, traditional culture cognition and understanding, and comprehensive abilities such as active thinking, active understanding, self-cognition, social cognition, emotional communication, interpersonal communication and aesthetic appreciation.