Halo 4 is expected to sell about four million units worldwide by the end of the week. This is an increase over Halo 3 which sold 3.8 million units worldwide in its first week and an increase over Halo: Reach which sold 3.7 million units.
If sales hold up for the rest of the week it would become the biggest launch for an exclusive console game in history. However it would not break the record for biggest opening, which goes to Call of Duty: Modern Warfare 3 which sold a combined 14 million units and 7.5 million units on the Xbox 360.
Halo 4 launched in North America and across Europe, with a release date in Japan on November 8.