Harmonix took to its blog to detail how — and if — songs from Rock Band 1-3 can be used within Rock Band 4.
The developer said it’s “hard at work trying to make Rock Band 3 exportable into Rock Band 4 and are shooting to have that available with the December 8th feature update.”
Harmonix strongly recommends that those new to Rock Band 4 buy Rock Band 3 before 30 October in order to export songs into Rock Band 4. It appears that licensing issues mean RB3 won’t be available for purchase through normal retail channels after that date.
“The Rock Band 3 export will be available for $14.99 [USD] for people that are verified purchasers of Rock Band 3 only,” the developer continued.
You can read more about song exports here.