You can make a transition to a “root” page in pretty much the same way in Ionic 4, the NavController provides these methods:
goRootgoBackgoForward
Which you can use to give Ionic’s router outlet a sense of direction (which can be important for animations especially). So, if you are going to a route that you want to be the root page you could just do this:
this.navCtlr.goRoot('/route');
Or, if you are using router link in your template:
<some-element routerLink='/route' routerDirection='root'>