What do you mean by four pillars and eight characters?

The four-poster eight-character fortune-telling book says: Xu is a prison guarded by dogs, and the sea is the Tianmen of the pig arch. The eight characters are empty, the prison is lawless, and there is no heaven. But fortune telling depends not only on the constellation, but also on luck and fleeting time. If you are unlucky, you will make up for this day's prison, and you will still have the opportunity to be invited by the police uncle to have tea and talk about life.