I have to add bootstrap 3 or 4 to my custom theme (magento 2.2.5)
I tried this:
[How to use bootstrap in my custom theme][1]
and this
[Bootstrap in magento 2 theme][2]
but these ways don't work with magento 2.2.5

  [1]: https://devdocs.magento.com/guides/v2.3/config-guide/bootstrap/magento-bootstrap.html
  [2]: https://magento.stackexchange.com/questions/195508/how-to-use-bootstrap-in-my-custom-theme/#answer-195553