The Four Worlds are widely correlated with the four letters of the Tetragrammaton, and Yetzirah, the third world, is assigned the letter Vav, read in this scheme as the connective letter, the point where the forming, patterning activity of creation links the higher worlds to the dense world of action below. The correspondence gives the descent from Atziluth to Assiah a second reading, as the successive spelling out of a single divine name across four levels of increasing density.
Kabbalistic teaching holds that Yetzirah, like each of the Four Worlds, contains a complete Tree of Life within it, its own ten sefirot, so that the astral or psychic world of the angelic choirs is not merely a midpoint on a line but a whole structure in miniature, receiving the pattern handed down from Beriah and passing its own formative structure into Assiah below. This nested repetition underlies the description of the Four Worlds as a ladder that can be ascended in meditation as well as descended in the account of creation.
Yetzirah corresponds in the soul scheme to Ruach, the moral and emotional soul, placing ordinary human feeling and ethical will at the same rung as the world of formation, the level Jewish mourning custom associates with a person's character as lived out in life.
Sources The ZoharTraditionally attributed to Moses de Leon with Encyclopaedia Judaica, 2nd editionMichael Berenbaum and Fred Skolnik (eds.)