The Trinity of Eastern Orthodoxy: the Holy Spirit flows from Father eternally. This is in contrast to the Catholic filioque, in which Son and Father are at the same horizontal level, from which the Holy Spirit flows, forming a triangle.
Orthodoxy prevails here, for the Trinity is a line, not a triangle.
The Father is Love, the Loving Beloved whose Love breathes a cosmos in loving submission. The Son is the Logos of Truth. The Son follows from the Father because Love is relational, Love has a direction: Love entails a Lover and a Beloved. So Love entails that God should breath the cosmos. The Son is Truth because only Truth can be come from the ruh (breath) of God. This action of breathing is the relationship of Holy Spirit from the Loving Beloved to the Loving Son. God’s most beloved lover is the Logos of Truth that God breathes in love.
But in turn, from our fallen perspective, the Beloved Truth is buried within us, sublimated within us, forms a body of difference, a logic of life upon which we can strive to attain Love in Cosmic Romance. (We sometimes call this sublimated body the Gifted Body, but more on that another time.) It is this Romance that makes our life into a journey, but also makes our love the strongest above that of other, immediately submissive entities.
That original breathed Word is still within us, but sublimated, so Truth walks with us, but we cannot see. Its sublimation makes our lives of difference possible.
So what does it mean to say the Spirit descends upon us? It is the mechanism of Truth. The Spirit descends upon us and Truth is breathed into us. Wisdom, Sophia, emerges.
But from the psychological perspective of the seeker, while this Wisdom appears seen as something that descends upon us, it emerges from within us through a (partial) actualisation of the Logos. The Logos is within the human form, but we never fully actualise it, unless we are a prophet. But we can all gather some Wisdom, because indeed the breath can be felt by everyone who seeks. So when we seek the Spirit, when we catch some of this breath, some Wisdom will emerge from us psychologically, and the effect is of a Shekhinah that descends upon us.
This experience is the Shekhinah-Sophia.
Why does a fourth come from the three? Why does the Shekhinah-Sophia follow? Because the Shekhinah is the creative potential that resides within human consciousness, because consciousness is language and the Shekhinah is speech. But the Shekhinah is also a divine emination of God, and God is above human consciousness. The Shekhinah is therefore the conduit between our minds and the Divine. The Spirit is the loving breath of God and is therefore separate from our minds. Logos is what we subsist on. But the Shekhinah-Sophia is what is emerges/is generated by/is produced/descends upon our human forms if we worship righteously. It is the Wisdom that comes from actualisation of some aspect of the sublimated Logos. The Shekhinah-Sophia is thus both the emergence/realisation of the Logos within and the descending/breathing of the Spirit above.
So seekers, on your Sabbath, on your Sunday, on your Jummah, run into the fields of wheat and seek out the descent/emergence of the Shekhinah-Sophia!