[url]http://www.examiner.com/article/halo-4-limited-edition-content-images-are-released[/url]
From the picture it looks like Halo 4 will come on two discs. Probably one for campaign and another for multiplayer.
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That's the Limited Collector's Edition. One disc is for the normal stuff (campaign, multiplayer, etc.) and the other is for the Halo 4 Forward Unto Dawn and extra stuff if I'm correct.