The four doors, schools of thought, or theories: 有 is the phenomenal world real, or 空 unreal, or both, or neither ? According to the Tiantai school each of the four schools 四教 in discussing these four questions emphasizes one of them, i. e. 三藏教 that it is real 通教 unreal, 別通 both, 圓通 neither; v. 有 and 空, and each of the four schools. In esoteric symbolism the 四門 are four stages of initiation, development, enlightenment, and nirvāṇa, and are associated with E., S., W., and N.; with the four seasons; with warmth, heat, coolness and cold, etc.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

（名數）一有門，二空門，三亦有亦空門，四非有非空門也。門者能通之義，依此四者，得入真性實相之記，故曰門也。##【圖像: T0044.JPG 刪減】

丁福保《佛學大辭典》

（術語）真言於曼陀羅之方位以東南西北次第，配於發心修行菩提涅槃之四法，故東門云發心門，乃至北門云涅槃門。今葬場四門之額，銘此四法，即由此出也。又四門者，為常樂我淨之四德，大圓鏡智等之四智也。世親攝論十五曰：「蓮華有四德：一香二淨三柔軟四可愛。譬法界真如總有四德，謂常樂我淨，於眾華中最大最勝，故名為王。」常為東方大圓鏡智，又名金剛智，金剛常住不壞，故比之以蓮華之香，香主遍至之德，橫遍十方，豎遍三世，是為常住之義，外四供中以香為東方，即此意也。樂為南方平等性智，南方自寶部，寶珠雨種種之財，人得而樂之，故比之以蓮華可愛之德。我為西方妙觀察智，阿彌陀名觀自在王如來，我即自在之義，故比之以蓮華柔軟之德，淨為北方成所作智，釋迦乘無住涅槃之理，出於五濁惡世之中，曾不染生死之妄法，故比之以蓮華清淨之德。見秘藏記鈔六。此四門有胎藏金剛之別，胎藏界之壇門稱曰金剛門，頂間畫金剛，金剛界之壇門，稱曰蓮華門，畫蓮華，是金剛為智，以理為能入，胎藏為理，以智為能入之標幟也。

丁福保《佛學大辭典》