I finally found some time recently to update the Yeoman generator for ArcGIS JS Apps to output a starter app for 4.0 as well as an option for Sass.
You can learn more about Yeoman here.
The generator will create a starter app for 3.x or 4.0 and also has all the tooling to create a custom local build of your application using the Bower release of the ArcGIS JS API. It also includes some optimizations or deployment to create a single file build and copies all resources or required external files into a release folder.
The starter apps are also in ES6/ES2015 and includes tooling to compile to ES5 via Babel.
This update also uses a neat little app architecture for 4.0 based on using Accessor to watch for property changes in the app.
For more geodev tips and tricks, check out my blog.