Advice sought on migration to Ionic 4

We have built an app that we would like to make available to desktop browsers as well as mobile devices. The app is an Ionic 3 app and is now in the Apple and Android app stores.

I knew there might be some compromises involved when it came to making the app function fully in a browser. The main issues we are having is the user experience when pressing Back, Forward and Refresh buttons i.e the way the app handles routing. Being an Ionic 3 app it uses the standard NavController push and pop for routing.

Our dilemma right now is whether we are better off trying to get the application working on the Web as an Ionic 3 application (i.e by possibly using Angular routing for the browser and NavController routing for mobile) or by migrating the application to Ionic 4 where Angular Routing could be used across all three platforms.

The app is fairly complex in size (40 odd components, over a dozen plugins and also uses NGRX Store for state management). On the mobile side it also supports iOS 9+ and Android 4.4+

After some advice really - would we be better off working on getting the Ionic 3 app working on the web as well as mobile or should we start to migrate now?

(also - does anyone know if Ionic 4 supports iOS 9 - the docs say it has been tested in iOS 10+ which doesn’t make me hopeful) ?