It seems that what matters most, as far as we’re concerned, are the three lowest levels of the soul, Nephesh, Ruach, Neshama; as Chaya and Yechidah are totally out of our experience. Only Adam and Moshe (and only as he stood on Mount Sinai at that) were said to have been granted the latter two.
Rav Y.E. Chaver writes in his Biur to Likutei HaGra that the Nephesh is actually lower than man, and that man’s essential self is found in the Ruach. The Neshama is above man, in the Heavens, and only sparks of it touch upon the Ruach.
Megalah Amukot (175) maintains in fact that Adam alone merited a Yechidah in full, which is the whole intent behind the statement, “Behold, the man will be like one of us and live forever” (Gen. 3.22). Moshe experienced it when he experienced the revelations of G-d’s “exalted wisdom” at Sinai, but it was taken away from him at the incident of the golden calf, when he only merited a Chaya.
Ohr HaChaim (Lev. 22:12) also holds that only Adam achieved a Yechidah, as did Limudei Atzilut (36), and Rav A. Shapiro of Dinov in Regel Yeshara (33c). And Rav Chaim Volshiner contends that even Adam only merited the faintest glimmer of Chaya and Yechidah (Nephesh Hachaim 1:15, 2:17).
The Zohar often speaks of the “Neshama of the Neshama”. What is that? According to HaRav m’Fano the combination of Chaya and Yechidah are subsumed under that title (Ma’amar HaNephesh 1:3).