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Harmonix now lets players export Rock Band 1 songs into Rock Band 4, at least to owners of the PlayStation 4 version in North America for now. According to its blog post, the company is still working on make the song export available for PS4 European users.
For Xbox One owners, the song export is currently not available, though Harmonix hasn’t forgotten about them, and is also “working to get these exports to Xbox One users as soon as possible.”
The song export will only work for gamers who purchased the Rock Band 1 export that has been available for five years. In other words, if a player doesn’t own the RB1 export, he or she won’t be able to export and download the songs into Rock Band 4.
Currently, 47 songs can be exported, while 8 more will be available some time later (see post for songs). Unfortunately, three songs will not make the export: “Paranoid” by Black Sabbath, “Run to the Hills” by Iron Maiden, and “Enter Sandman” by Metallica.
[Source]: Harmonix: How do I export Rock Band (1) into Rock Band 4?.