We are already starting to see it happen. v4 plugins are being reported on https://isstrapiready.com and likewise to our own market (Which isn’t public yet, but will be soon).
Migration guide from v3 to v4 will come in January (we simply didn’t have time with the holidays coming up and lots of people on vacation)
I’d say early to mid Q1 we should see the bulk of our users either already on v4 or moving to it quickly. Most currently active plugin maintainers have either already started working on v4 ports or completed them already.