Direct from Harmonix, this is the Rock Band 4 compatibility chart. If it’s not listed, they haven’t tested it yet, so the list looks like it’ll be updated in the future. Also, it appears the Legacy guitar controller adapter (required for Xbox 360 guitars on the Xbox One) is going to be a valuable item if you find it at a garage sale or thrift store. Nobody is going to know what the heck it is! Check out the main picture of Rock Band 4 on amazon to see what it looks like.
Lastly, it appears that the value of our old guitars will hold up because the initial pre-orders of Rock Band 4 is for the stand alone software, meaning people will need to order last-gen guitars to use it!