According to a NeoGaf’s user, Halo 4 have already been out before its release date. Here’s an image below:
There was two videos uploaded on YouTube, but Microsoft has claimed the copyright for these videos.
This is about gameplay: http://www.youtube.com/watch?v=q7-oRL0jwws&feature=youtu.be
This is the disc loading of Halo 4: http://www.youtube.com/watch?v=e31FjkdIPHc&feature=youtu.be
Both videos are no longer available due to a copyright claim by Microsoft Corporation
Article from Gamersyndrome.com