Rock Band 4 the game, all of its DLC, and the previously released plastic instruments you might have already will work on the PlayStation 5 and Xbox Series X.
Developer Harmonix confirmed this in a blog post, stating that all of this will work on day one when the consoles release in November. Harmonix couched its statement with phrases and words like “we believe” and “should” because it cannot guarantee compatibility in all cases for all instruments.
Electronic drum kits and their various adapters should work as well, Harmonix said. In terms of DLC songs, all of them will work on the PS5 and Xbox Series X. Additionally, save data carries forward, but only on the same console family.