What does the four pillars of pure Yin mean?

Pure Yin Bazi means that the attributes of the Heavenly Stems and Earthly Branches in your Bazi are all negative

For example, the Heavenly Stems: A, B, C, and D. . . A is Yang wood, B is Yin wood, B is Yang fire, and D is Yin fire. By analogy, the same goes for the earthly branches.

There are many opinions about pure yin in the horoscope. In terms of marriage, there is a saying that a person with a pure yin in the horoscope should match a person with a pure yin in the horoscope.