They are described in several places. They are four spirits of god.

Zechariah 6:5
And the angel answered and said unto me, These are the four spirits of the heavens, which go forth from standing before the Lord of all the earth.

Revelation 4:6
And before the throne there was a sea of glass like unto crystal: and in the midst of the throne, and round about the throne, were four beasts full of eyes before and behind.

Ezekiel 1:22 And the likeness of the firmament upon the heads of the living creature was as the colour of the terrible crystal, stretched forth over their heads above.

The sea of glass or crystal is this creation. The four spirits are the foundations of it, they hold apart the expanse. This is the four rivers in Genesis.