Rock Band 4 is now released and available to the general public, on Xbox One and Playstation 4. The following information is taken from an ealier post I wrote on the matter and will be updated as time goes on.
First and foremost, if you as for a code from EA for ROck Band 2 to Rock Band 3 export, it will still work (see the below posts)
I can also confirm a few things from watching a pre-release Harmonix stream, which unfortunately I do not believe has been archieved in one single place yet, in addition to my experience with the game.
Campaign, whilst playable to a degree, is not narrated/doesn't have sound effects from what I saw.
Voting during songs doesn't have to be done, you can wait until the end of a song to vote (sometimes you won't be able to vote during a song either so this makes sense.)
Crowds are different to previous games (I can't recall seeing a single recycled crowd thus far in the game's trailers or actual footage).
Quickplay mode is just one song at a time as before. Shows mode is the equivalent of an endless setlist, where you choose songs as you go through. There is no actual setlist option yet though.
I believe you can start a band whilst in the song list - other people just press start/green/buttons on their instrument/controller whilst the other person is in there and they can join.
I don't claim to be a full authority on this game, but post any RB4 questions and it might be possible to find an answer.
Also, the tutorials for freestyle solos are completely playable as they are all spoken.
For achievements that I've used DLC for or that I can remember what songs I've picekd, I'll try and list them here (this won't be an extensive list)
[dlc] To obtain gold stars on a song I played "My Own Eyes" by Weird Al on expert vocals.
[dlc]To get 4 overdrives as a vocalist in one song, I used Still Alive by Jonathan Coulton.
As before, keep posting questions and I'll see if I can find answers.