Up to Ionic3, it was closely packed with Angular.js. So probably the navigation jobs were done by some Angular.js functionality. I can be wrong since I have not worked with Ionic or Angular.js before.
Ionic4 will be framework agnostic, written with Stencil.js and compiled with Stencil.js Compiler to Web Components standard components.
I wonder how navigation tasks will be handled, and by whom? Does Ionic4 have routing capabilities? Or it does not and developer's client-side router of choice must be used, especially I wonder the use case with Stencil.js since it has a router too. For example, how it should be used for a tabbed application Mark up and styles by Ionic, and routing with Stencil.js?