Coordinate System and Using Custom Maps Verification

Hello, I am using Cesium and wanted to ensure that the coordinate system used is WGS 84 with EPSG:4326 (Not Web Mercator). Is this the same for the globe and the 2D tiles? I am using the arcgis imagery provider with everything projected in 4326, just wanted to ensure that Cesium does not change this upon projection and that everything should project fine.