For an interface, understand the user, their task, the interface's previous and next steps, and the content and functionality needed to support the user.
Why to use? 4-corners helps teams understand how an interface fits into a user's journey through a system, so they can optimize the content and functionality and improve the user's experience.
When to use? Use 4-corners any time you need to sketch or review and interface.
The following resources provide more information about using this method.
Collaborative Product Design by Austin Govella
Chapter six provides detail on variations and alternatives for using this method.