I like CC BY-SA 4.0. IMO, it's better than 3.0. I'd prefer to use it. I've chosen to use 4.0, instead of 3.0, on projects of my own. However, my preference doesn't affect the current situation with respect to existing content on Stack Exchange.
Stack Exchange doesn't have the right to unilaterally change the license of previously submitted content.
Beyond what might exist in the CC BY-SA 3.0 license, I've seen nothing that indicates Stack Exchange has the right to change the license for previously submitted content. No version of the TOS which I've read grants the additional right to Stack Exchange to re-license prior existing content, or, in the future, to re-license content submitted under the current TOS.
If you believe such a right was granted outside of the CC BY-SA licenses, I would appreciate knowing what wording in which document you feel grants that right.
The primary issue is migrating from CC BY-SA 3.0 to CC BY-SA 4.0. Unfortunately, Stack Exchange unilaterally shifting the existing license on older subscriber content from 3.0 to 4.0 isn't permitted, or at least that's the opinion of the Creative Commons, who wrote the licenses.
Creative Commons has a 4.0 upgrade guidelines document, which says in part (emphasis mine):
Upgrading from prior versions to 4.0
- Who owns the rights?
Adaptations of existing content:
- Who owns the rights to the original?
- If the contributor, then can license new contributions to an adaptation under 4.0 but original contributions remain under prior version unless express permission to upgrade is obtained.
You can change what happens with respect to old contributions moving forward by mutual agreement with the user that submitted the content (i.e. each question, answer, comment, tag excerpt/wiki, and edit). In other words, you can get permission from each author to re-license the content they submitted, but you can not do so unilaterally.1 You could change the TOS such that new use of the site relicenses old contributions by that user, but that's something you should discuss with your lawyers.
However, as a practical matter, you will never be able to get permission from every contributor to switch to the new license for all previously submitted content (e.g. users who never return to the site), which means you must deal with mixed licenses in some manner.
Was this change planned and intentional?
This announcement says that you "inadvertently introduced a point of confusion". This can be read many different ways. One is "the person who was updating the TOS mistakenly typed a 4 instead of a 3 when linking to the CC BY-SA license." The wording you've used makes it sound like the change was a mistake that you just decided to go with. It doesn't sound like you put planning into making such a change.
The fact that the change from linking/saying CC BY-SA 3.0 to CC BY-SA 4.0 did not happen close to seamlessly in all places on the site (or at least most) argues that this was not a planned change. In particular, that CC BY-SA 4.0 has been linked in the TOS for more than a year and you are only getting to making an announcement and other on-site changes now, really makes it look like the change was, at least initially, just a mistake.
Please, run this by your lawyers.
If this was not planned and/or you have not yet run this by your lawyers, I strongly recommend that you inform your lawyers of the current situation and ask for their advice.
My belief is that by making this change you've created a quite complex situation, which is difficult to resolve. My understanding is that the resolution recommended by the Creative Commons requires changes to how you display subscriber content (i.e. indicating the applicable license individually for each question, answer, comment, tag excerpt/wiki, and edit).
You might be able to get away with just making it clear as to the date that the license changed, but doing that will require every user who copies content from Stack Exchange to individually determine which license is being used for each portion they copy.
I am not a lawyer. This is not legal advice. This is just my interpretation from reading the license, the current TOS, my memory of the old TOS, etc. I strongly recommend that you seek the advice of your lawyers.
- You can change your TOS unilaterally, but that doesn't change what agreements are in place from the past. Changing those existing agreements (i.e. the existing licenses) requires action on the part of the other party (i.e. your users).