We recently changed our copyright on All The Tropes to include the option for CC BY SA 4.0 licensing. In fact, here's the relevant excerpt from our Copyrights page:
All The Tropes uses a Creative Commons Attribution-Share Alike 4.0 International
license (CC-BY-SA). All text on the site is licensed under CC-BY-SA
unless otherwise noted; images may or may not be under this license --
you'll have to check each image individually. This is part of our Terms of Service.
All CC-BY-SA content is also available under a Creative Commons Attribution-Share Alike 3.0 Unported license, i.e. the previous revision of the same license. This is a compatibility mode with Wikipedia and Wikia.
To explain what this means, we have dual licensed our content for redistribution. CC BY SA 3.0 still applies to everything, but now you can license to the newly created CC BY SA 4.0 when redistributing the content as well. All content on our site since the change is now available under both licenses, and for compatibility with sites that don't have CC BY SA 4.0, we have retained 3.0 licensing for places like Wikia and Wikipedia.
This is legal by the following terms from the Creative Commons website:
The Adapter's License You apply must be a Creative Commons license with the same License Elements, this version or later, or a BY-SA Compatible License.
Basically, any derivative of the CC BY SA license is covered, so dual licensing is as legal as the licensing for CC BY SA and the GDFL is on Wikipedia.
However, this applies to redistributed material. Porting 4.0 licensed material to All The Tropes (Orain) is another story.
If you wrote it under 4.0, you will have to redistribute it to us with the understanding it is being relicensed with the CC BY SA 3.0 license as well. If the material is someone else's work under 4.0, they must give consent for this to happen, or we cannot accept the material in question.
We elected to add 4.0 licensing to our material to allow others to take advantage of the newer licensing if they choose, and we are retaining the older 3.0 licensing for compatibility and redistribution to websites that do not have support for or do not wish to use the newer licensing.
Please contact the ATT administration if you have any questions or concerns about this change.
P.S. - Our plan for any future modifications to this change is to follow any changes done by the Wikimedia Foundation to their own, assuming they make any changes to their own licensing regarding the 4.0 CC license.
Also, neither I nor anyone else on the ATT staff are lawyers and we cannot give legal advice on this matter, though we tried to make sure this change is as legal and non infringing to our site and its contributors, and are prepared to retain legal services should any questions or concerns arise in the future concerning this change.