Why did the GR"A institute a count of eight times 36 years, and not four times 72, four times the name of G-d?
Perhaps the eight signifies the number of "months" of the "pregnancy". The Arizal explains in Ets HaChaim, Heichal Zeir Anpin, Sh'ar רפ"ח Nitzotzin, Perek 5, that "birth" occurs at eight "months", and one "day", which is insignifant in that it does not change the count of רפ"ח.
The months are 36 years each. The one day corresponds to one year: 5501 + 8*36 + 1 = 5790.
Note that this marks the end of the four Partzufim, of Malchut and Zeir Anpin, of Chochma and Bina. After this, Malchut can rise to Keter.
