This is the achievement icon for completing all of the rituals on shadows. (This achievement doesn't appear in the xbox360/ps3 versions for some reason) Seeing as the shadows ritual circles are now being considered for the Impossible EE, thought I'd bring this up.
From what I translated they're (left to right) Envy, Greed, Pride and Lust, referencing the sins of the map's cast. Could also just be a straight reference to the sins and has nothing to do with being a key or anything like that but who knows.