Just set your themeSystem setting to ‘bootstrap4’ and make sure to include a Bootstrap 4 stylesheet AND a FontAwesome stylesheet.
Unlike Bootstrap 3 which comes with Glyphicons, Bootstrap 4 does not come with a defacto icon set. In order to create a connector for FullCalendar we had to choose an icon set, so we chose FontAwesome. To learn more, read the FontAwesome Getting Started page as well the FullCalendar bootstrapFontAwesome setting.
In other news, Scheduler v1.9.3 has been released.