There are Guidelines and they contain a Paragraph "Dealing with mixed-version (e.g., 4.0 and prior versions) content".
I'll just quote that here:
- Always mark which license version applies to which content. CC has published best practices.
- In many cases, a work will have multiple rights at play because there is more than one contributor to a work. This often means there are multiple license versions applicable to a work. In those cases, reusers must comply with all relevant license versions when reusing the full work.
- For, example, reusing a 4.0-licensed translation of a 3.0-licensed essay would require complying with the conditions of both versions 3.0 and 4.0. This means attributing the original author as specified in v3 and attributing the translator as specified in v4.
- CC has published guidance about what differs between versions. As a practical matter, compliance with a later version of a license is typically compliant with the earlier license. There are certain exceptions with respect to attribution. A comparison of the attribution requirements is here. (The definition of NonCommercial is unchanged from prior versions, and the scope of ShareAlike has only expanded, so looking only at the later license should not be problematic vis-a-vis those license elements.)
- Note: These same considerations apply when dealing with any two different CC license versions and/or types. They are not unique to 4.0.
What this means is that Wikipedia could't just upgrade all content to CC BY-SA 4.0.
But they could decide that new content has to be licensed by the contributors as CC BY-SA 4.0.
They would then need a system to track exactly which content is licensed under which license and give this information to users of the website.