I'm trying to find the equivalent in 4.1 to the 3.x's MapImageLayer.addImage method, e.g.
mapImageLayer.addImage(new MapImage({ extent : extentForDataUrl, href : someDataUrlString }));
Is there any update to this? I am running into the same issue.
Update as of July 2017: "Coming soon not to be confused with the 4x MapImageLayer which is the 4x version of the 3x ArcGISDynamicMapServiceLayer)"
Reference: Functionality matrix | ArcGIS API for JavaScript 4.4
