As relayed by trustworthy NeoGAF user SolidSnakeX, the fourth Guitar Hero title will expand the series' offerings to include drum and vocal gameplay in addition to bass and guitar. It is not known if the controllers for Harmonix's multi-instrument music title Rock Band will be compatible.
The song creation functionality will somehow allow users to customize everything but vocals, though the specifics of this remain unclear. Users will apparently be able to upload and share songs on all platforms, and will even be able to create their own album art.
Four bands have been confirmed thus far for the game--Van Halen, The Eagles, Linkin Park and Sublime--with Neversoft noting that all songs within the game will be master tracks provided by record labels.
In addition to song creation, Guitar Hero IV is said to place a strong emphasis on player and instrument customization, in part due to the recent legal troubles with guitar maker Gibson.