You need NodeJS and Ruby 2.1+ installed.
Sass is good to have and required for some tasks.
Install NPM and Bower dependencies:
npm install
bower install
Run Gulp:
gulp
Install Bower packages and they'll be included in the HTML automatically (most packages, anyway).
Bootstrap 4 beta will be included from src/css/lib/bootstrap.min.css and src/css/lib/bootstrap.min.js.
Source code for Bootstrap can be found in src/css/lib/bootstrap/4.0.0-beta/.
You can customize it by changing src/css/lib/bootstrap/4.0.0-beta/scss/_variables.scss and recompile.
Run the following commands first as necessary (might have to use sudo for gems):
cd src/css/lib/bootstrap/4.0.0-beta/
gem install bundler --verbose # if you don't have it already
bundler install --verbose
npm install --verbose
Then, run this to recompile after every change:
npm run dist