Fallout 4 won't be shown for at least a year or two. They still have Skyrim to support, and they may try to make another "expansion-pack" after Dawnguard.
Probably the only thing that Bethesda could show for Fallout 4 at this point in time is concept art, scribbles on paper, and some rough ideas, charts, etc...
concept art, scribbles on paper, and some rough ideas, charts, etc... are the best, most exciting things about a new game though!