Harmonix has announced that it's working on an update for Rock Band 4 that should enable purchasers of Rock Band 3 to export its tracks to the latest version of the game.
Due December 8th, and costing $14.99 to export the tracks, you'll need to be a verified purchaser of Rock Band 3 to be able to port your tracks to Rock Band 4. Basically, it seems that your original purchase will be verified electronically, meaning that you'll need to have bought and played Rock Band 3 for the export process to work. I assume that means it'll be checking your profile to see whether it includes Rock Band 3 data.
Why do this? Well, Rock Band 3's soundtrack is one of the series' best – featuring 83 exceptionally diverse songs from five decades, including Jimi Hendrix' Crosstown Traffic, Deep Purple's Smoke on the Water, the seminal Freebird from Lynyrd Skynyrd, Queen's Bohemian Rhapsody, The Power of Love by Huey Lewis and the News, Walk of Life by Dire Straits, Rehab by Amy Whitehouse, and The Hardest Button to Button by the White Stripes. And, of course, the many, many more.
But what about exports of the first two games? Apparently, Harmonix is, "still working on supporting previously exported Rock Band titles in Rock Band 4 and hope to have that content available for those of you who have already exported into Rock Band 3 later on." No date has been announced as of yet, so we'll just have to be patient and wait to see when the first two discs will become exportable – hopefully sometime early next year.
Incidentally, "any tracks from the Rock Band 3 disc with authored guitar solos will feature upgraded Freestyle Guitar Solo and Freestyle Vocal features," so get ready to rock out with a killer solo to Freebird!
Below is Harmonix' offical chart showing the current state of disc exports in regards to Rock Band 4.