Four siddhantas (catuh-siddhanta, 四悉檀):
(1 loka-siddhanta) shijiexitan 世界悉檀 Worldly siddhānta. Preaching which accords to the conventional understanding of the world.
(2 prthagjana-siddhanta) gegeweirenxitan 各各爲人悉檀 Siddhānta for each individual. Preaching according to the abilities and levels of understanding of the people listening.
(3 pratipaksa-siddhanta) duizhixitan 對治悉檀 Special application siddhānta. Preaching aimed at destroying strong defilement or evil karma of certain beings.
(4 paramartha-siddhanta) diyiyixitan 第一義悉檀 Siddhānta of supreme truth. Preaching of reality as understood by the Buddha himself.
also here (Tiantai): The Four Siddhanta
Nagarjuna on the four siddhantas: part 1 part 2 part 3

