So far, version 4 of the ArcGIS API for JavaScript only supports a limited number of layers compared to v3.x. This will undoubtedly change over time. Some of the most interesting changes are listed as follows:
GraphicsLayerFeatureLayerMapImageLayerVectorTileLayersGroupLayersSceneLayers
The GraphicsLayer is just a clear layer that overlays the map and allows you to mark up the map with your own custom graphics. In this sense, it is very similar to what you've seen in v3.x. However, in v4 you can add graphics of different geometries to the same graphics layer and this has an important knock-on effect. Because the GraphicsLayer supports different geometries, you must define any renderers or popups against single graphics and not for the graphics layer as a whole. If you need this functionality, consider using a feature layer instead.