As recently as yesterday, gamers have been receiving invites to join the Halo 4 Beta through Twitter upon mentioning the word "Halo". With that said, the beta invites are not actually real and are simply a clever attempt to steal user details from unwitting gamers.
David Ellis of 343 Industries, which is developing Halo 4, has confirmed that the invites being sent out are indeed a scam and warns gamers from clicking the links provided within them.
If you see a page claiming to allow you to sign up for a Halo 4 beta be advised, IT'S A FAKE. youtube.com/watch?v=7qKcJF…
— David Ellis (@DavidEllis) January 17, 2012
Similar scams have gone out in the past, with the Diablo 3 beta, among others. These beta invites, done through unofficial Twitter accounts, are simply social engineering attempts to steal user information for the purposes of fraud and identity theft.
Should the Halo 4 beta commence, it'll be done through Microsoft and 343 Industries.