“Ionic 4” is obsolete, and you don’t really want to be targeting it. Let’s say for the sake of discussion that you’re trying to bring a project from being an “ionic-angular” (v3) project to an “angular” (modern) project.
Right. The work that used to be done by ionic-app-scripts is now delegated to Angular CLI, so you can refer to its documentation for more information.
Is this of concern for you simply because of unfamiliarity, or are you dependent on such a file existing?
Per-component styles go in files referenced by the styleUrls in the @Component decorator of the component. Global styles go in global.scss. Theming variables go in theme/variables.scss.