The Secret of 4

There are several valuable trinities in theology and philosophy. There are also several trinities that are apparently complete, but contain a covert fourth among the overt three. Many trinities simply lack the presentation of an unsaid, subtle and often hidden fourth element of the set. The overt three and the covert fourth occurs because of the transcendent character of the fourth aspect.
One of the most profound archetypal and ancient concepts illustrative of the hidden fourth aspect is the set of The Three Wise Monkeys, with the missing covert Fourth Monkey, Fear No Evil.

​ Be The Fourth Monkey

Go Fourth, Forthrightly!

Whereas 3he 4ourth Initiation focuses on internal individuation, The Matrix of Four presents a similar dynamic with more of a philosophical, observational, and problem solving direction.
The Geometry of Energy is an exploration of meditation and subtle energy via the four dimensions of Sacred Geometry.
108 Steps to Be in The Zone reveals the fourth aspect in the overt trinity of the sacred number, 108.