As of “Magical Mystery Cure”, there are four princesses in Equestria: A Princess of the Sun, A Princess of the Moon, A Princess of Love, and A Princess of Friendship (as stated in “Twilight’s Kingdom Part 2”). However, is it possible that the princesses actually represent the four elements?
Princess Celestia – Fire
Princess Luna – Water
Princess Cadence – Air
Princess Twilight Sparkle – Earth
Princess Celestia, being the one who raises the sun, would naturally represent “fire”.
Princess Luna, who raises the moon, would be considered “water”, as the moon controls the tides.
Princess Cadence, Princess of Love, would be “air”, an element that represents the soul. In addition, Cadence was a pegasus before she became an alicorn.
Princess Twilight Sparkle is “earth”: nurturing and stable. We’ve seen her motherly side when dealing with Spike and, although she sometimes struggles with her stability, she is the most rational one in the group.
If the princesses really do represent the four elements, it makes sense that there would be four princesses. It is believed by some people that bringing together all four elements causes balance. A balanced kingdom is a good kingdom.