The operative word there is "some" — Rock Band song export is currently live only for PlayStation 4 users in North America. Harmonix said it is still working on bringing the feature to Xbox One worldwide and PS4 customers in Europe.
As Harmonix has noted before, you'll only be able to get your Rock Band tracks into Rock Band 4 if you already exported those songs to Rock Band 3. The Rock Band export offer has expired, so it's impossible to buy it now.
The original Rock Band disc on PlayStation 3 and Xbox 360 came with 58 tracks. The export process currently provides 47 songs; three tunes — Black Sabbath's "Paranoid," Iron Maiden's "Run to the Hills" and Metallica's "Enter Sandman" — have never been exportable due to licensing restrictions. But the export pack includes plenty of classics and party favorites, such as Weezer's "Say It Ain't So," David Bowie's "Suffragette City" and The New Pornographers' "Electric Version." The remaining eight songs, which were bonus tracks in Rock Band, will be available at a later date.
The export process itself works the same way it did for Rock Band 3, which became a supported title in December. On PS4, you'll have to individually download all of the songs through Rock Band 4's in-game music store. Harmonix is planning to add support for importing songs from Lego Rock Band and Green Day: Rock Band in the future.
For more on Rock Band 4, read our review.