This is just my own collection of Bootstrap 4 enhancements. I recommend putting the SCSS snippets into an .scss/.sass file that imports bootstrap.scss at the top, but you're welcome to use them however you'd like.
I also included enhancements_compiled.css gist, which is a compiled version of these enhancements using Bootstrap 4 default sizes & colors, for those who don't re-compile SASS from BS4 source.
The buttons.css gist contains the following enhancements :