A supposed Halo 4 multiplayer video showed up on YouTube over the weekend. Submitted by Skid Kid, the video is a VHS tape of someone playing what appears to be Halo 4 although it's hard to tell due to the VHS being played on a CRT TV while being recorded by a non-HD video camera. There's also no audio other than incredibly loud rap music. The gameplay is definitely not from previous Halo games, but it's hard to tell specific due to the quality of the video.
At the end of the video, the person playing the VHS tape takes out the video, shows an envelope that has some words on it "halo_4_beta_Tom_Morello." There's a link to the user's Twitter account that was created on May 26th has had nothing but posts of weed with the exception of comments regarding the Halo 4 video.
Games have been trying to dissect the info available as more "leaked" content has been making its way to various social media. Below are images that were uploaded to Reddit by a user who's account was deleted.
Here's something else interesting, a user on Reddit, batnah, compiled proof that would support that this is a viral campaign and then a post that says it's not. The user, however, has only been on Reddit for 2 days (around the time the video was "leaked"), and has only commented on Halo 4 topics, providing a compilation of evidence.
We'll have to see how this unfolds over the next coming weeks, especially with E3 coming up next week.