Spark
Broken
Fallout New Vegas was announced quite a while before release, however the first trailer was released only 8 months before release. Also, Fallout 3 has a long period from Announcement and release, but also had a new engine. Skyrim was announced with a trailer (similar to Fallout 4), and was released 11 months later. However Skyrim also had a new engine I think. I doubt Fallout 4 has a new engine, although that's yet to be seen.I'm fairly certain it'll be a summer 2016 release, or even later than that.
I believe they announced that a big project when New Vegas was released, and that project later turned out to be Skyrim.
New Vegas is from late 2010, and Skyrim is from late 2011. I believe Fallout 4 will be rather similar. Released about a year after reveal.
Either way we should find out more at E3.