Can I include material from Wikipedia in a Stack Exchange post?

Question

Now that Stack is on CC BY-SA 4.0, is it possible to include material from Wikipedia, which still uses CC BY-SA 3.0? I have no desire to be a lawyer, but I'm given to understand that the two licenses are not fully compatible. Presumably, short excerpts from Wikipedia are acceptable under Fair Use, but it's a judgement call how long a Fair Use quote can be, and I'd rather have a definitive answer.

I tried searching for this here and on meta-SO, by the way. All I found were questions and answers written before the license change (and the answers from when Stack was on v2.5 were not particularly promising).

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In trying to figure out why someone might have been motivated to downvote this question, it occurred to me that they might have thought I was trying to make some kind of oblique point about the licensing change. That was certainly not my intention.

I was writing an answer for which it would have been useful to include a snippet from Wikipedia, and it occurred to me that it might not be allowed. When I couldn't rapidly find a confirmation that it was OK, I figured that it would be better to ask a question and get an on-record answer than to try to figure it out myself, particularly since I'm not a lawyer. I hope the accepted answer is useful for that purpose.

Answer 1

Yes, you can. See Compatible Licenses, which explicitly states that (emphasis by me)

Your contributions to adaptations of BY-SA 3.0 materials may [..] be licensed under BY-SA 3.0, or a later version of the BY-SA license.

So what you do is creating a derivative work that is licensed under a later version which is 4.0. However, attribution, i.e. a link to the original Wikipedia content, is still mandatory.

Unfortunately, the other way is not possible. You cannot take content posted after May 2 2018 from StackOverflow and include it in Wikipedia. That's a real drawback of the latest license change here.

Answer 2

You can create an “adaptation” of anything in Wikipedia and post here as CC BY SA 4 is “a later version of [CC BY SA 3] with the same License Elements”.