On December 1st we will no longer be able to export the Rock Band 3 track list into Rock Band 4 anymore. This allows the import all all 83 songs into Rock Band 4.
On Xbox you don't even require the Rock Band 3 disc simply purchase the following DLC and you will be able to import the songs into Rock Band 4 as DLC.
This makes it fantastic value, especially if you skipped Rock Band 3.
On PlayStation you require the Rock Band 3 disc and a PS3. You then and have to go through the prompts in the in game store to export your songs.
Any disc will work for the export its not a one time use.
Rock Band 4 and all DLC will work on both XSX, XSS and PS5.
If you have any idea you might play the game in the future don't regret missing out. Instruments can be picked up cheap on online marketplaces and it has karaoke mode for the non gamer. Great fun for the entire family.