And the truth is that, as taught by the ARI, nine Sefirot of Arich Anpin are called essential roots of Atzilut, while all the other Partzufim are branches of Arich (see Etz Chayim, Shaar Arich Anpin ch. 2). Accordingly we must consider them two levels: Atika Kadisha (the “Holy Ancient One”) is Arich Anpin, while the other four Partzufim are together called the “Name of Atika”, as explained in the Idra (ibid.).
Thus Arich is one single AV whose four Yuds bring forth AV, SaG, MaH and BaN. For we see that there are these four names of AV, SaG, MaH and BaN, which are respectively Abba and Imma, Zeir and Nukva, but we find no name for Arich Anpin, and sometimes AV is Arich Anpin. The explanation is that there is a name AV that is the essential root of all the other four names, which issue forth from its four Yuds. For we already know that each Yud indicates a Havayah, as stated in the Tikkuney Zohar. And this overall AV is Arich Anpin, and from the four Yuds emerge AV, SaG, MaH and BaN, which are Abba and Imma, Zeir and Nukva.