Wikipedia on Four Zoas (vide "Albion"):
"In the complex mythology of William Blake, Albion is the primeval man whose fall and division results in the Four Zoas: Urizen, Tharmas, Luvah, and Los/Urthona. The name derives from the ancient and mythological name of the British Isles (see Albion).
The long, unfinished poem properly called Vala, or the Four Zoas, expands the significance of the Zoas, but they are integral to all of Blake's prophetic books.
The division of the primordial man is found in many mythic and mystic systems throughout the world, including Adam Kadmon in cabalism and Prajapati in the Rig-Veda."
Not much really, but enough to get the idea. I just want to add that Urizen & Ahania represent Reason, Faith, Certainty; Luvah & Vala: Passion, Love; Tharmas & Enion: Sensation, Coherence, Receptivity; Urthona & Enitharmon: Instinct, Creativity.
(All of them are prone to Fall, very Wakean indeed: so Faith turns to Doubt and Tyranny; Love to Rebellion and Revolution; Coherence to Chaos; and Creativity to, erm... obviously to Poetry and Prophecy. I'm not quite sure how this is a "Fall". As I've said I don't understand too much of this. And there are two more entities involved: Orc is the fallen form of Luvah; Los the fallen form of Urthona.)