How can I use Bootstrap 4 while using Flask for Python 3?
There is a fork called Bootstrap-Flask that is up to date.
That said, however, it is not a good idea to use either Flask-Bootstrap or Bootstrap-Flask. These are very thin packages with a small layer of indirection that primarily serve to obscure your understanding.
Instead, I recommend using the starter template provided by Bootstrap 4, which will always be up to date to whatever version you require using their CDN.
If you stick with Bootstrap, you will soon want to build your own Bootstrap anyways so you can do things like add custom themes. Hacking Bootstrap with CSS instead of using their SASS customization scheme is painful (I did it for a while). The build process is well documented and simple enough, just a few installations required.