"Halo 4" just so happens to be Microsoft Studios' best-selling game of all time in the United States, after it recently surpassed "Halo 3" for sales during each game's respective launch year, according to the team. While "Halo 4" hasn't passed the total unit sales of "Halo 3" - it has only been out for under a year whereas "Halo 3" has been out for almost six years - it does make the most recent Halo title the "best-selling" game for a single year from Microsoft Studios.
The title will likely be the last Halo game produced for Xbox 360 as Microsoft Studios and 343 Industries focus on developing a new Halo title for the Xbox One, and although a trailer was shown at E3 2013, it's not known if the upcoming game is a direct sequel or a separate entry in the saga. "Halo: Spartan Assault" is also about to be launched, although it's a spin-off title developed for Windows Phone and Windows 8/RT with a third-person, top-down perspective as opposed to the first-person viewpoint of the Xbox titles.
Source: Microsoft | Image via Microsoft