No, actually, it's more expensive that Rock Band... Rock Band is only $170, but GH4 is $200, as far as I know.
Can't decide whether I will get Rock Band when it comes out or wait for GH4. GH4's song creation and the ability to freestyle with drums, as well as the raised cymbals, that's going to be pretty sweet... but it's going to be a real long time before that comes out. Don't know if I can wait with Rock Band out and all.