StoryMaps
The new StoryMaps uses ArcGIS API for JavaScript 4.x. Classic Story Maps continues to use ArcGIS API for JavaScript 3.x.
Web AppBuilder
Web AppBuilder currently uses ArcGIS API for JavaScript 3.x for apps built for web maps. Experience Builder, currently in Beta, uses ArcGIS API for JavaScript 4.x
ArcGIS Dashboards
ArcGIS Dashboards currently uses ArcGIS API for JavaScript 3.x. ArcGIS Dashboards Beta uses ArcGIS API for JavasScript 4.x.
Map Viewer
Map Viewer (classic) uses ArcGIS API for JavaScript 3.x. Map Viewer Beta uses ArcGIS API for JavaScript 4.x.
Configurable Apps