I use bs3 and love it, however my plan is not to upgrade to bs4, it's to slowly pull out and replace anything that could be modernised over time. The risk of breaking something is too great.
That's slightly hyperbolic. I don't think they intend for the primary use case to be for people to upgrade in place, like updating a nightly Firefox, but rather to start new projects with the new version. But previous big upgrades have been largely a bunch of search and replaces. There's various tools that people have written to automate this. Again, I wouldn't blindly run it and expect everything to work, but it's far, far less than a complete rewrite.
We all do tend to do a redesign of our sites and apps eventually and that the correct time to make the transition.