The ArcGIS 4.0 Beta has had three releases so far and has introduced plenty of new features and functionality.
This video covers a series of samples that have been created to demonstrate and explain some of these new features. It is incredibly useful for developers moving the 3.x API to 4.0.
The repo of demos can be found here.
These samples include demos of Accessors, Collections, Maps and Views separation, Promises, fromJSON methods, advanced use of SceneViews camera, LayerViews, and of course one of my favorites, View Models!
Get more familiar with the 4.0 API and be ready to build applications to meet your needs and the needs of your users!