Harmonix has recently put up the official instrument compatibility chart for Rock Band 4 in all it’s glory. Over at the Harmonix forums, HMXCRISIS states –
“A quick note: The Legacy Game Controller Adapter for Rock Band 4 on Xbox One seamlessly supports most wireless guitar and drum controllers from previous editions of Rock Band for Xbox 360®.
The adapter supports up to four controllers simultaneously and enables players to use most of their existing wireless guitar and drum controllers from Rock Band 3, Rock Band 2 and most third party wireless instrument controllers for the Xbox 360.”
Well, there you are!