This may be a late reply but Halo 4 is based on Reach's engine. However there is a lot of improvements done on the engine itself such as dynamic lightings and better particle effects which makes the game running on the 360's limits (the highest possible graphics for the 360, not to mention Forza 4, Battlefield 3 and Crysis 3). And about the ragdoll (Animation, flying bodies, etc) the series have been using the Havok physics engine (except Halo CE) and I believe Halo 4 is using the same thing. Same engine also means sooner release date! Why would 343 want to make a new engine causing Halo 4 to be released on 2013?
