I've always wondered about that. I mean there are six elements of harmony and six bearers, why wouldn't there be four other bearers when Celestia and Luna first took control of the elements? Think about it, after defeating Discord the sisters other bearers ruled Equestria, prolonged exposure to the elements turning them into Alicorns (that would be the reason why Celestia keeps the elements locked away instead of with their proper counterparts.) Over time the power corrupted the other element bearers and one by one Celestia banished them to the stars until finally Luna succumbed to their power. But instead of banishing her sister forever she imprisoned her on the moon, afterwards relinquishing control over the elements so she would not become corrupted. "The stars will aid in her escape." Four stars, for other bearers.