Here's a somewhat cryptic passage from the SD, p. 337 (Primordial Substance and Divine Thought)

In the Sepher Jezireh, the Kabalistic Book of Creation, the author has evidently repeated the words of Manu. In it, the Divine Substance is represented as having alone existed from the eternity, boundless and absolute; and as having emitted from itself the Spirit.* “One is the Spirit of the living God, blessed be Its name, which liveth for ever! Voice, Spirit, and Word, this is the Holy Spirit;”† and this is the Kabalistic abstract Trinity, so unceremoniously anthropomorphised by the Christian Fathers. From this triple One emanated the whole Kosmos. First from One emanated number Two, or Air (the Father), the creative element; and then number Three, Water (the Mother), proceeded from the air; Ether or Fire completes the mystic four, the Arba-il.‡ “When the Concealed of the Concealed wanted to reveal Himself, he first made a point (primordial point, or the first Sephiroth, air, or Holy Ghost), shaped into a sacred form (the ten Sephiroth, or the Heavenly man), and covered it with a rich and splendid garment, that is the world.”§

Footnote(s) ———————————————

* The manifested Spirit; Absolute, Divine Spirit is one with absolute Divine Substance: Parabrahm and Mulaprakriti are one in essence. Therefore, Cosmic Ideation and Cosmic Substance in their primal character are one also.

† “Sepher Jezireh,” chap. 1, Mishna ix.

‡ Ibid. It is from Arba that Abram is made to come.

§ “Sohar,” I., 2a.

It gets repeated, somewhat clearer, on p. 447 - Westcott added the term in the Theos Glos

Arba-il (Chald.). The Four Great Gods. Arba is Aramaic for four, and il is the same as Al or El. Three male deities, and a female who is virginal yet reproductive, form a very common ideal of Godhead. [w.w.w.]

The term seems to be adapted from Thomas Inman:

http://books.google.ca/books?id=HIEBAAAAQAAJ&q=arba#v=snippet&a...

HPB actually develops the concept more fully in Isis (p. 170 et seq.) relating it to the Pythagorean upper Tetraktys.

http://www.theosociety.org/pasadena/isis/iu2-04.htm

In a subsequent post, I will endeavor to show some modern research in Near East religion and the Kabbalah that tends to corroborate the notion.