I'm unable to sign in to my profile on the PlayStation 4 when using my legacy music game controller.
Keep in mind that this is only affecting those who are attempting to play Rock Band 4 with a legacy music game controller. In order to navigate around this, please sign in to your PlayStation Network ID before launching the Rock Band 4 game. If you attempt to sign in to your profile after the game has launched, you will be forced to sign in as a Guest. You will need to restart the game if this occurs. If you want to sign in to your PlayStation Network ID while using a legacy music game controller, you must sign in before launching Rock Band 4.