If you look at the crest in the tower by the message place, there's four different symbols encased. Three of the symbols corrolate to the 3 classes. Lastly, the 4th symbol on the bottom right (the one with diagnol lines) does not have any relation to anything really. Could it be it relates to a 4th class? Perhaps the symbol relates to a past ally? Maybe there's no relation at all and they just needed to fill the space. What do you think?
Edit1: link to picture http://fc05.deviantart.net/fs70/i/2014/235/4/2/destiny_crest_by_ropa_to-d7w8d4u.png
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Screenshot?