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.
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.