Our friends at The Hooniverse have your first ever look at the 2.3 liter turbo four cylinder in the 2015 Mustang. Head over there to check it out.
Why does this exist? Under boost, I guarantee it will consume as much, if not more fuel than the V6, with notably worse throttle response and less reliability.
The only plus I see is cruising MPG—but I'd gladly trade a few mpg (say 32 vs 28) on the highway for the sound, reliability, and throttle response of the V6.