A curated list of awesome Bootstrap 4 resources:

• Bootstrap 4 - Official Bootstrap 4 site.
• bootstrap-rubygem - Bootstrap 4 Ruby Gem for Rails / Sprockets and Compass.

• Bootstrap Kit - All you want from Bootstrap 4 but don't have in Bootstrap 4 (yet).
• Bootstrap Float Label - Pure CSS implementation of Float Label pattern for Bootstrap 4.
• Bootstrap Layouts - Generic Bootstrap 4 layouts. Use as: HTML boilerplates -or- Jekyll _layouts -or- Codepen templates.

• Bootstrap Kit - All you want from Bootstrap 4 but don't have in Bootstrap 4 (yet).
• Material Design for Bootstrap - Powerful and free UI KIT for Bootstrap 4. (source)
• Anchor Bootstrap KIT - Anchor is a free Bootstrap UI Kit with flexible, ready to use UI components which will hep you build websites faster.
• AdminKit - AdminKit is a premium UI Kit & Dashboard Template that comes with hundreds of UI components, forms, tables, charts, pages and icons.

• ng-bootstrap - Bootstrap 4 components, powered by Angular 2 and written by the angular-ui Team. (source)
• bootstrap-vue - Twitter Bootstrap 4 Components For Vue.js 2
• reactstrap - Simple React Bootstrap 4 components
• bootstrap-ie8 - compatibility fix for IE8 and IE9

• On Hackr.io - Bootstrap Tutorials and Courses

• Awesome Bootstrap 4 - A curated list of awesome Bootstrap 4 resources.