For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
Given that Media-A-Team are using Fabrik in their commercial products for customers one has to assume there will be some move to J4 Fabrik. How this J4 version is made available is another matter.I would be happy with a confirmation that it's "work in progress". Timelines vary and these days that is especially true. Still, other developers have made statements regarding their J!4 readiness, JEvents for example. So when I come here, and I don't see a new forum thread, or news blog post discussing upcoming version plans, it makes me wonder if it is actually "work in progress".