GH4 will sell sell, if just solely on the name recognition and compatibly with existing guitars for PS3/Wii users. People do not want two sets of guitars, just like they do not want two sets of drum kits.
GH3 sales = 10.65m
RB sales = 2.29m
There are at least 8 million GH3 owners who do not own Rock Band. A lot of them own a second guitar for multiplayer. That makes GH4 the cheaper option for upgrading to a "band" game. ( for Wii/PS owners)







