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).
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.