According to a recent interview with Activision CEO Bobby Kotick, Guitar Hero IV could be set to follow in Rock Band's footsteps by expanding out from the guitars-only approach and add drum kit and microphone peripherals to the mix.
It's unclear, however, whether Kotick was referring specifically to Guitar Hero IV or referring to the series in a broader sense. It would seem a bit strange, though, to call the game Guitar Hero IV if it will be focusing on drums and vocals as well. Naming issues aside, it will be interesting to see how Guitar Hero IV competes against MTV's increasingly popular Rock Band.
On a side note, Kotick also hinted that Guitar Hero IV could feature "local content" from "local bands" - meaning it could well become a platform for indie bands to get more exposure and make more money via digital distribution.