Stencil is the compilator use behind the scene to build Ionic v4. If you don’t want to, you could totally forget about it respectively ignore it if you are not interested. Ionic will still provide ionic-angular.
Finally, the migration from v3 to v4 should be way easier as from v1 to v2. I didn’t tried but had a look at the documentation, it definitely looks like search and replace mostly. I’m still not sure about the router system, if it will be a must to rewrite it, probably not, but except that doesn’t look that hard. Also I recently upgraded my web app from Angular v5 and Rxjs v5 to both v6, was easy