Online multiplayer has long been missing from Rock Band 4, being a hugely requested feature from fans as you can imagine.
And it's finally coming later this year as confirmed by Harmonix in this video:
However, there is a catch. This feature is not going to be available to everyone. Instead, it has been confirmed in this blogpost that it will be part of an announced expansion pack for the game. There is more coming to that pack, which will be talked about at E3, but the nitty gritty means this: you will have to pay for online multiplayer.
This seems like a bizarre way to split up the community, and I'm sure this might irk some who expected it at the launch of the game. We will have to see what else comes in this expansion pack to truly judge, but reaction to this should be 'interesting'.