I just read in Game Informer that Guitar Hero 4:World Tour officialy has all the band instruments; Guitar, Bass/Rythm, Vocals and Drums. I found a picture of the drums and Game Informer has gave us some news about it but nothing on the other instruments.
Appearently NeverSoft wants to keep the NEW guitar under wraps. It will have downloadable content on every system (probably not PS2 though), it will be much more organized and have more songs and bonuses to download. You will be able to make your own songs and send them out over WiFi, but you won't be allowed to put vocals in the songs (probably because of copyright issues). The game will include band legends like Ledzepplin, Nirvana, Metallica, etc and it will feature real guitar heros in the boss battles.
The game will have the biggest set list ever in a music game, and will be endless with the In-Game Studio. The game will have new changes, like if you get stuck on a song, you can switch to a different difficulty without even making a new band so you can just pickup where you left off. You will also be able change instruments if you get stuck as well, but they will do different things in encore. If you play an encore as guitar or bass you'll still play a fifth song, but using drums you will play a drum solo. The lights will fade and center on the drums, the fireworks will go off all while you hammer away on a totally improved drum solo!
The game will then rate you on your drumming rampage. You will also have full customizable characters, from what cloths they wear to what color their face paint should be (if they have any). You can make your own custom guitars as well with complete customization of everything on it. You can make custom band logos, tattoes for characters, album art and even make a band logo to put on your drum front. There isn't a official release date yet, but it will supposedly be released between late October and December.