Number four has deep relation to the season and the direction.

Four protective gods of the four directions in ancient China
are called "shi-shin" in Japanese:

Sei-Ryu (Blue Dragon) ------------ East
Suzaku (Chinese Red Phoenix) ----- South
Byakko (White Tigar) ------------- East
Genbu (Black Turtle) ------------- North

In Buddhism, the Four Devas are called "Shi-Tennou" in Japanese:

Jikoku-ten ------------------- East
Zouchou-ten ------------------- South
Koumoku-ten ------------------- East
Tamon-ten (Bishamon-ten) ------- North

There are many phrases prefixed with "shi" (four) especially in
Buddhism. For example,

Shi-tei (four truth)
Shi-ku (four sufferings)
Shi-dai (the greatest four)
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