I'm fairly certain that's to represents the four variances on the Lunar Boss.
As I already stated in my findings on the trailer, The boss has a variance depending on where you've spawned it from. The four planets represent four elements, Each of which the boss can attain depending on where it's from.
Jungle for example, Has an electrical variance Don't ask me why they didn't go with Fire, Earth, Water, and Air, While the forest has a fire variance. Corruption/Crimson had the Shadow/Air variance, Which leaves either the Snow, Desert, or Ocean Biomes for the remaining variance. Alternatively the Snow and Forest Biome could share the same element leaving the last one for the Ocean/Desert.
The biomes definitely have something to do with the boss type that spawns, We'll find out just how much when the update goes through.