Several attempts to establish a system of categorizing the Buddhist Canon had been made as a means to synthesize diverse Buddhist doctrines. However, … there was not a single theory that could perfectly arrange the whole teaching of the Buddha systematically and justifiably until the emergence of Chih-i. He has formulated a systematic classification P’an-chiao, and has made it the primary recourse of justification for the consistency of all the Buddhist doctrines as a whole. His system of classification consists of three aspects.

1. First, the Buddha’s whole teaching career is arranged in five chronological periods that correspond to the five flavors of dairy products, upon which the doctrines that are expounded in these periods are introduced.
2. The second aspect is the method the Buddha uses to expound his doctrines in terms of sudden, gradual, indeterminate, and secret, which are related to the five periods.
3. Finally, based on the chronological arrangement, the Buddha’s teachings are categorized into the Four Teachings. As we have mentioned above, the Fourfold Teaching is discussed throughout the Hsüan-i (Profound Meaning of the Lotus Sūtra) , and it is so significant that it is the primary principle in systematizing Buddhist doctrines and practices. (Page 136)

 
The Profound Meaning of the Lotus Sutra: Tien-tai Philosophy of Buddhism