As web technologies and new features become supported in browsers, sooner or later, the tendency and the direction of the Sencha company will be to merge Ext JS and Sencha Touch into a single framework. So, the most logical thing is to call it "Ext JS 6", but no worries! What you learn in this book will be very useful for the next version because Ext JS 4's and 5's classes, coding style, extending classes, and patterns such as MVC or MVVM will be available in version 6 as well. So, it will be much easier for you to understand what all this is about.
This is what you need to keep in mind:
Wait for a stable release
Check out the documentation for changes, new things, and what will be required for the migration of version 5 apps to version 6
What Sencha CMD tool's version 6 can handle and what it cannot