The internet seem to take it as granted that Fallout 4 is being announced at Bethesda's first E3 press conference ever. While I think it's very likely, I don't think it is a certainty. I can see a world where we don't catch a glimpse.
That being said though, here is a new rumour to mull over. This comes from German website PCGames, (via NeoGAF) who cite that their sources say a 20-30 minute demo will be shown at the end of their show. It also says that the game will not launch in 2015 as some have guessed.
However, take this with a HUGE grain of salt. The website seems to quote well known leaker shinobi602 as their source, but over on NeoGAF he spoke up and said:
Why is my name in there lol.
Games could be being played here by him or the website, but it won't be too long until we find out.
I am leaning towards this not being true. What with Shinobi seemingly denying it and the fact that a 20-30 minute demo sounds incredibly long, I just don't see it. I could be wrong though. I'll let you know if anything changes.