Halo 4 uses Old Engine

Halo 4 will use the same engine as Halo 3 but with several improvements.

Microsoft announced at the end of their E3 press conference the new Halo 4, and that's not everything. With it a whole new Halo trilogy was announced, so a fifth and sixth sequel are for sure.  
Rumours say that Halo 4 is being developed for years now and will be released at christmas time 2012. Phil Spencer, vice president of the Microsoft Studios, talked in an interview with IGN about the new title and revealed that there would be no new engine included but the old Halo 3 engine with some improvements by 343 Industries. If you expected much better graphics, you could be disappointed. 

Phil told IGN also that if the HD remake of Halo 1, called "Halo: Combat Evolved Anniversary", would be a moderate success, the chances of a remake of Halo 2 are very good. Further Microsoft announced their plans about the releases of new Halo titles. There will be no new Halo game every year which could hint to the new Xbox 720 with Halo 5 or Halo 6 as launch titles, to demonstrate the power of the new console. Information will be revealed at the E3 2012.

Source: halo4.org