Oh, btw—Bootstrap 4 will be in SCSS. And if you care, v5 will likely be in PostCSS because holy crap that sounds cool.
— Mark Otto (@mdo) April 23, 2015
This is great news!
If you have watched the Advanced Bootstrap Tutorials then you already have an advantage. If not, now is your chance to get started!
The full playlist is below and starts with the basics of using the grid mixins.