In MCTB Daniel writes the following in the chapter
What Went Wrong:
How do I know that solely content-based practice won’t produce insight? Because there are only Three Doors to ultimate reality, that’s why, and they are utterly unrelated to content, though they can be found in all content if the content aspect is ignored. (Actually, there is sort of a fourth door that is accessible to very realized beings, see the Appendix.)
In the appendix I can't find a reference to this or more likely I have overlooked it some how.
So, what is the fourth door?
Answer