There was a huge article in the latest GameInformer magazine about it, from what I read (it was my friend's) it will be released in October '08. I heard from another site that it is confirmed to have a song by Van Halen on it. Oh, and GH: Aerosmith is NOT GH4, it's only an "extension" of GH3 just like Rocks the 80s was an extension of GH2.
Another website whose name I already forgot
By now you should have your very own June issue of Game Informer in your hot little hands – and what an issue it is. This month’s magazine brings you an exclusive look at three of the most anticipated games currently in development. First, our cover feature of Guitar Hero 4 reveals Nerversoft’s plan for the future of the franchise – vocals, drums and the ultimate form of creative control with the ability to craft your own songs.