Harmonix also released a new FAQ for Rock Band 4. I'm not sure what exactly is new, but they confirm that just about all wireless instruments will be compatible and will release a compatibility chart soon. Guitars with just five buttons will still support the Freestyle Solo feature, but obviously it'll work with the limitations that the guitar can handle. They're looking into a way to do RB3 exports, but aren't guaranteeing anything just yet. Standalone instruments won't be available until early 2016, so bundles are the only way to get the new instruments at launch.