A fir tree signifies the natural principle as to good. The fir tree, pine tree, and the box tree, as in Isaiah 60:13, signify the celestial natural things of the Lord's kingdom and church, consequently, such things as relating to external worship. A fir tree also signifies natural truth superior. A fir tree, as in Ezekiel 31:8, signifies the perception of the natural man.

(References: Arcana Coelestia 4014)

The number "four" in the Bible represents things being linked together or joined. This is partly because four is two times two, and two represents the ultimate linking between our desire to be good and our understanding of truth. We can also see this in the fact that most buildings are rectangular, with the four sides linking together to make a whole. We also divide directions into four -- north, east, south and west -- and talk about the "four corners" of something meaning all of it. So our special thinking naturally looks at four sides as linking together into a whole.

(References: Apocalypse Explained 417; Arcana Coelestia 1686, 9103, 9601, 9767, 9864)