Perhaps the fifth race (the Glimmer) isn't being revealed because they are the most important. I think there are a few possible scenarios. Either: A) the Glimmer brought the other races into the Sol system, whether indirectly or as a part of an allegiance (like the Covenant). B) the Glimmer appear in later into the story, and either serve as a common enemy (like the Lambent, Locust, Human conflict in Gears of War), or force the different aliens to work together, with or without the humans, to fight them. C) the other races are entering the Sol System to get away from the Glimmer (similar to how humanity was pushed into other systems to get away from the Flood) D) the Glimmer were the mysterious cataclysmic event that nearly wiped humanity out. When the traveler sacrificed itself, they were knocked out, but they are starting to return, and are more powerful than all the other alien races. Perhaps the Glimmer are more powerful than the other races because they can disguise themselves, and take the shape of other lifeforms... It would certainly explain [url=http://www.bungie.net/pubassets/615/dark_patrol_desktop.jpg]this image[/url], and also explain their name. They may be called the Glimmer because that's the only way to tell them apart from those they are copying, their method of disguise isn't perfect, and the impostors will "glimmer" a bit to give themselves away to the wary-eyed. We really won't know for sure what they are or why they haven't been revealed until they [i]are[/i] revealed, but those are my best guesses.