Porting BS3 → BS4 can be easy or hard. It took me about a day to port my first site but things have been smooth sailing since then. BS3 is still officially supported (the devs thankfully aren't repeating their unceremonious dumping of BS2 this time around) so unless there is some pressing reason for you to update you don't have to.
Are you sure?
https://github.com/twbs/bootstrap/issues/20631
> Stop all work on v3—today. The open issues, split dev setup, and more holds us back from focusing entirely on v4. I'll close all remaining v3 issues and milestones. Any new changes to v3 will be sporadic and highly irregular.
BS3 is so widely used at this point that point that I think it is fair to call it 'done'. Browsers aren't going to stop supporting the features it uses any time soon.
> When we shipped Bootstrap 3, we immediately discontinued all support for v2.x, causing a lot of pain for all our users out there. That was a mistake we won’t be making again. For the foreseeable future, we’ll be maintaining Bootstrap 3 with critical bug fixes and documentation improvements. v3 docs will also continue to be hosted after v4’s final release.
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.
Not that there's a need to rush it; I really appreciate Bootstrap's "it's ready when it's ready" stance. Personally I also like that the BS devs had the balls to rewrite their grid system based on flexbox this late in the development cycle, rather than hanging on to something you're not convinced of and focussing on getting a release out the door.
To those criticizing Bootstrap, here are my reasons why I like Bootstrap:
BS was incredibly useful for getting me back into contemporary/responsive CSS after a longer period of absence from webdev. I ended up overwriting many things, but that's why it's called "Bootstrap" after all I guess.
Bootstrap is also very useful as a baseline CSS in larger projects with multiple teams developing a uniform-looking web site.
And you can get a ton of themes for relatively little money (though not everything on offer is great of course). You can also contract an agency or freelance designer to make a BS theme for you, with a straightforward workflow.
Also, you can get relative good and targetted StackOverflow etc. advice on your daily CSS problems.
And, when BS4 finally is released, you don't have to redo your complete site to take advantage of it (or at least I hope so).