The 4.1 documentation is here.
The 3.18 documentation is here.
The 4.1 release includes some great new features in the MapImageLayer, 3D scenes and custom Popup capabilities. There’s even some foundation being laid with the ability to save WebScenes for more Portal integration with the JSAPI.
The 3.18 release is starting to include features we’ve seen in the updated ArcGIS Online such as the new FeatureTable, which is incredibly powerful. The VectorTileLayer has had an overhaul and some improvements implemented.