DLC is a big draw for such an expensive title - it helped Rock Band immensely as well. I've seen or heard so many complaints about lack of DLC support in GH3 or Rock Band for Wii that people refused to buy them, or forswore buying any more GH series games. In addition, I don't know about you, but I don't have room for a new drum kit in my apartment with each iteration, or have the money to plunk down over $350 on two games every year.
In addition, Activision is always going to be a step behind Harmonix, since they don't care to invest in innovation unless someone else does it first - it's your traditional big corporation. Unless they can prove that they can one up the experience by that much, reasons for justifying spending so much on GH 4 won't be there, and there's nothing to suggest that it will so far.