Note: I can only speak for 360 -> Xbox One
I was concerned buying stuff for RB3 wouldn't work now that the licensing deals were all done, but I just tested it out with a song pack. Three songs for 240 MS points ($3) that are currently on sale in RB4 for 1.99 each.
Over 1000 songs are on sale for 80 points each ($1), and all of them are showing up at 1.99 in RB4 (Nothing is on sale in RB4 right now). Basically it's a 2-for-1 at half price, since you're getting it for both systems.
You're also able to have both your 360 and Xbox One online at the same time with the same account. I'm comparing libraries, and the purchase was made while both games were running. No waiting, kinda great.
Just a heads up, all.
Edit: Just bought about $30 worth of discounted songs on 360 as individual songs, packs, and albums, they all work on Xbox One.