Any DLC that you purchased on PlayStation 3 / Xbox 360 will be available to download on the next-gen console from the same manufacturer. However, DLC purchased on next-gen consoles will not be available for download on previous gen systems.
For maximum clarity:
Buy on PS3, own it on PS3 and PS4
Buy on PS4, own it on PS4
Buy on Xbox 360, own it on Xbox 360 and Xbox One
Buy on Xbox One, own it on Xbox One
If you have further questions about how DLC is handled on next-gen consoles, please see the link below:
http://forums.ubi.com/showthread.php/924044-Rocksmith-2014-PlayStation-4-amp-Xbox-One-FAQ
Current Status:
Xbox One (12/23)
All legacy DLC apart from the import tool is available for sale. Anyone who bought DLC on Xbox 360 as single Songs or Packs should be able to download some of that material as Songs, but the permissions required to do that are not yet in place for all users. We have seen song permissions make progress; however, it's still somewhat slow, so we do not have a hard date on when we expect all legacy content permissions to be sorted. We are working with Microsoft on this, but we aren't expecting any progress during the holiday break.
PlayStation 4 (12/23)
All legacy DLC is available for sale in the US, including the import pack -- if you have already purchased it on PS3, you'll see it in the Packs menu as the Rocksmith Disc Import Pack. If you have not already purchased it on PS3, we hope to have an update for you soon.
Please keep an eye on the link below for updates:
http://forums.ubi.com/showthread.php/924044-Rocksmith-2014-PlayStation-4-amp-Xbox-One-FAQ