In this section Blake has recourse to an old, old truth; the four elements were basic in ancient philosophy ; for example Empedocles' philosophy is best known for being the origin of the "cosmogenic theory of the four Classical elements." (wikipedia).
Read Gates of Paradise as a pdf file in the Library of Congress.
Digby on pages 24-30 casts light on Blake's intentions in this part of our poem:
2. Doubt self-jealous, Watery folly:
Water is the province of Tharmas (related to the function Jung called sensation).
3. Struggling thro' Earth's melancholy:
Earth belongs to Urthona (Los) - the center of intuition which comes from 'down below'. Urthona in the person of Los works at bringing the creative seed out of the earth into the active plant.
4. Naked in Air, in shame and fear; Urizen belongs here; note the inscription ("on cloudy doubts and reasoning cares"). Urizen has come forth with the law, but he's sure as heck nervous about it (we read in Night 2 of The Four Zoas
" No more Exulting for he saw Eternal Death beneath
Pale he beheld futurity; pale he beheld the Abyss"
(FZ2-23.14-15; E313). In Blake's poems the stars (as shown here) represent Urizenic order.
5. Blind in Fire, with shield and spear; Emotion! Love, Desire, Hate, you name it. Luvah of course!
Perhaps this series led to Blake's choice of the four zoas, the primary basis of his psychology. Carl Jung also picked up on this reality, having read Blake as well as Empedocles and Blake's other readings.
Jung came up with sensation, intuition, thinking and feeling.