I guess that will be just a development announcement, right?
This is the interesting question. *puts on detective hat*
Skyrim:
announced December 11, 2010
released on November 11, 2011
bethesda game studios is the developer of ES series, bethesda softworks is the publisher. Important because alot of idiots still think its the same developers as dishonored, ESO etc. etc.
anyway the last thing FO4 dev did was last skyrim dlc release in feb 2013. in april 2013 they announce moving to to next project.
I think its safe to assume pre production of FO4 would start after skyrim release in 2011/2012, only content guys left on skyrim dlc. then full production in 2013.
full production length 2.5 years. which isn't much. however the gap between fo3 and oblivion was quite short because they didn't focus that much on upgrading engine/graphics.
whereas with skyrim they spent alot of time upgrading engine graphics, it seems unlikely they will bother doing that again until ES6
I instead fo4 tech could be very similiar to skyrim and it so could come out fall 2015. While this means fo4 won't look much better than skyrim on PC, it obviously means fo4 will look much better than skyrim on consoles because they are a new generation.
the equivalent graphical leap jump of skyrim from ps3 to ps4 is enough to satify the ignorant masses, they don't need to spend more on tech since they know fallout will sell like crazy either way.
TLDR: FO4 could come out this year and look worse than witcher.