I'm one of those who want something to do other than quickplay. My wife and I want a reason to play all my songs again. We're finding that since we completed the tour mode (didn't take long) in RB3 we only ever play the game when some DLC comes out that I buy. We play the songs once and only if they are our favourites do they ever seem to get played again and that's not often. Therefore we need an RB4 to add more depth to the tour mode. I'm sure many feel this way too.
I think there is enough interest in the game to make a refined RB3 for release this Christmas if 1.5m sales as quoted earlier is true (even if many were at discounted price) as less time on development would be needed.
I also agree somewhat the TAG that if they don't come out with one this year they are never likely to come out with another RB game again. If not forever than at least for this generation of consoles. A resurrection of the genre is a possibility for the next generation but it is likely to lead to everyone having to start all over again with DLC and possibly hardware.