It sounds like they're going to do a short trailer at E3, but it will be more of a tease for the bigger reveal at Gamescom. The Gamescom reveal will be the start of the marketing campaign for the full game.
Yeah, Bethesda needs to show Fallout 4 at E3, the fans will probably storm their E3 stage if they don't, lol. Fallout 4 was rumored for both previous E3's and both previous VGA awards shows and didn't show up at either. I don't think Fallout fans can handle another disappointment. Bethesda needs to at least announce it with a CGI teaser, even if they aren't ready to show gameplay yet.
A CGI trailer would be somewhat of a copout, but it would be better than nothing.
It would be awesome if it was revealed and gets a Holiday 2015 release date. They'd probably "win" E3 right there.