According to various forums around the web, the new Game Informer has GH4 as its cover story, and shares some details.
*GH4 will (Like Rock Band) include all 4 instruments (Bass,Guitar, Drums, Vox)
There is scanned pictures from the GI mag of the nw GH drumset, and tbh it looks pretty nice. It has 3 drumpads, and two cymbal pads that come up above the drums like a real set. Those 5 pads plus the bass pedal means there will be 6 colors as ooposed to the normal 5 RB's drums use, as well as guitar on both games.
(Unless the two cymbol pads share a color, meaning you hit which ever cymbol you feel like)
GH spokesmen said in the article they wanted to makt their drums quieter then their competitors. For anyone who has ever played Rock Band knows, the drums are epic. But they are noisy.
*The new guitar will have some type of input that will "Change the way you play GH" according to deveolpers. As to details on what that means we dont know.
*They have confirmed Van Halen, Sublime, Linkin Park and The Eagles will all have songs in the game, as will At The Drive In and Def Leppard.
*there will be a "Create a song" feature, which wont include vocals. But will rival Apple's Garage Band software as far as allowing you to create and track music through your system, with hundreds of effects and such.