Microsoft says sales of Halo games have topped 50 million units since the series debuted on the original Xbox with 2001's Halo: Combat Evolved.
The platform holder said in a press release distributed on November 6, Halo 4's release day, that the series had clocked up 46 million unit sales, helping to generate $3 billion since launch.
And yesterday, a month after the game's release, it said: "To date, more than 50 million copies of Halo games have been sold worldwide, driving more than 5 billion hours of gameplay by people connected to Xbox Live."
That puts Halo 4's first month sales at around four million units.
Earlier this week, Frank O'Connor, Halo franchise development director at 343 Industries, told CVG that the studio is gradually reallocating its teams to work on the next major Halo project, which is expected to be Halo 5.