In an interview with OPM (Original Playstation Magazine), Infinity Ward’s animation lead Zach Volker went into some details about the new game engine that will be powering the upcoming game "Call of Duty Ghost".
Here are a couple quotes from the interview to summarize:
Infinity Ward has been significantly upgrading its existing game engine for the past 2 years. While it can be claimed that the end result is a new game engine, it is not a new engine built from the ground up. Which means that Ghost will be running an upgraded game engine that was most likely used in "Call of Duty: Modern Warfare 3"
Source: Playstation Official Magazine - UK