A quaternity or quaternion often has a 3+1 structure, in that one of the terms composing it occupies an exceptional position or has a nature unlike that of the others. (For instance three of the symbols of the Evangelists are animals, and that of the fourth, or St. Luke, is an Angel.) This is the ‘Fourth’, which, added to the other three, makes them ‘One’, symbolising totality. In analytical psychology often the ‘inferior’ function (i.e., that function which is not at the conscious disposal of the subject) represents the ‘Fourth’ and its integration into consciousness is one of the major tasks of the process of individuation. In Christianity, the focus is on the trinity (the Father, the Son, and the Holy Spirit), however, Jung viewed this as inadequate as it purposely left absent the evil or shadow counterpart – the Devil – which must be integrated by the trinity in order to become One; good cannot exist without evil and vice versa, once the two concepts merge, all concepts shatter and is replaced by non-duality, or Oneness.
C. G. Jung: “The quaternity is an archetype of almost universal occurrence. It forms the logical basis for any whole judgement. Id one wishes to pass such a judgement, it must have this fourfold aspect. For instance, if you want to describe the horizon as a whole, you name the four quarters of heaven…There are always four elements, four prime qualities, four colours, four castes, four ways of spiritual development etc. So, too, there are four aspects of psychological orientation… In order to orient ourselves, we must have a function which ascertains that something is there (sensation); a second function which establishes what it is (thinking); a third function which states whether it suits us or not, whether we wish to accept it or not (feeling), and a fourth function which indicates where it came from and where it is going (intuition). When this has been done, there is nothing more to say… The ideal completeness is the circle or sphere, but its natural minimal division is a quaternity.” (Psychology and Religion West and East, Collected Works, Vol. 11, p. 167.)