Now this really is a surprise to me. As if the fact that you can import all your previous Rock Band 3 DLC into Rock Band 4 wasn't enough, Harmonix have to pulled another huge feature out of the bag.
Despite being Next Gen only, Rock Band 4 will be compatible with older instruments from the Xbox 360 and PlayStation 3 days. IGN report that the PlayStation version will simply require you to plug in the existing PS3 dongle to get going, while the Xbox One version will have a "low cost" adaptor.
This really is a killer feature for this game as it makes the game affordable once again. The is key in not burning out this genre again, so it is good to see Harmonix really pushing to please fans and set up precautions to avoid the fall.