For those of you that are interested in the Rock Band 4 legacy adapter, they were just released for pre-order on amazon at a very large discount! They are regularly priced at $25 USD, but there is currently a coupon code discounts it $15 making it only $10.
The adapter can be found here. Use coupon code "RB4ADAPTER" at checkout.
I am not sure if the offer is available outside of the US.