A super simple rails app to show how to implement bootstrap 4. These steps worked on macOS. Please, submit a Pull Request if you find something needed for other OS.
1) Gemfile
# Add these gems. Rails is not including jQuery by default.
# You need to include it to use all bootstrap options.
gem 'bootstrap'
gem 'jquery-rails'
2) app/assets/javascripts/application.js
jquery3, popper, and bootstrap are the ones for bootstrap
//= require jquery3
//= require popper
//= require bootstrap
//= require rails-ujs
//= require activestorage
//= require turbolinks
//= require_tree .3) app/assets/stylesheets/application.scss
@import 'bootstrap';4) Run bundle and restart your server