Here's the wikipedia article, but I just picked the major points to list here.
http://en.wikipedia.org/wiki/Guitar_Hero_IV
- New "beginner" difficulty, as well as the normal easy/medium/hard/expert
- drum star power activated by hitting both cymbals
- bass guitar has a 6th note in note track, played by strumming w/o any fret buttons
- highly in-depth character and guitar customization
- old GH controllers can be used
- new guitar controller has 5 buttons similar to RB's solo buttons, to be used to alter sound in-game and in the custom song creator
- custom song studio (in ALL versions) to create and share songs online (PS2 cannot share custom songs)
- Wii drums/guitar become wireless with Wiimote slots similar to GH3
- mic is wired
- Slash and Tom Morello battles return, as well as POSSIBLE rumored characters...
- rumored battles/playable characters include:
Ozzy Osburne
Ted Nugent
Sting
Jimi Hendrix
Zakk Wylde
Hayley Williams (of Paramore)