The sad thing is that I think there's a really good chance that RB is going to fail because it doesn't really satisfy any particular market for the game.
For new buyers, or people who don't have their equipment like yourself, the price of entry is just ridiculous. $249 for a "full band" (I put it in quotes because you only get 1 guitar) is a heavy investment, and unlike when these games came out for last gen, there's no other games that this equipment will work with and vice versa. At least last gen you could use Guitar Hero stuff for Rock Band and vice versa. Also, this has been universally heralded as the weakest song lineup of all the Rock Band games. I realize that music taste is subjective but when basically everyone agrees that it isn't that good, it's probably not as good. This is due in no small part to the fact that Harmonix doesn't have MTV backing them anymore, so they lost a LOT of negotiation power for music licenses. So to really get a pleasurable experience, not only do you need to pay over $100 just to get the game with an instrument (Unless you decide to just get a USB microphone and use the game as karaoke) but you also have to pay more money for DLC to get songs that you like. Not very compelling.
For people like me that still have all their equipment (including old consoles with DLC) the package is less compelling. For the old instruments, unless you are in the Playstation Family, you have to pay extra to get the adapter to use your old equipment. There still isn't full compatibility of old stuff, although that may change later. But even if we do get full equipment compatibility, what we DON'T have is full DLC compatibility. While it looks like all the stuff from the RB Music store works along with songs from the spinoffs like AC/DC, Greenday, Lego, etc work, exported stuff from Rock Band 1 and 2 do not, and 3 had no exporting option as it didn't come with any codes for exporting. Also, various DLC packs that they sold in disc format don't appear to work either. So for me, playing RB 4 means that I lose access to at least 220 songs that I can't currently use in the game. 1 & 2 may change, but unless they figure out a way to export with RB 3, then I'm out at least 83 songs.
So if I get RB 4 and just want to play that, I'll have to pay more money, and get less songs. REALLY not very compelling.
And finally, Guitar Hero Live provides a more unique experience, has arguably a better guitar in terms of frets, AND costs $30 cheaper for the game and guitar.
While I do give Harmonix kudos for trying to resurrect the franchise, I can't help but think that this is basically a doomed project. They don't provide very compelling reasons to start from scratch or come back, especially if you still have all your old stuff. Of course this could always change with better instrument compatibility, importing options, and a price drop, but I don't know how many people will still be interested by the time that comes. You need to strike when the iron is hot.