In the 4.x API the MapImageLayer class has a new purpose and they have yet to implement a class in the 4.x API that replaces the old MapImageLayer functionality.
In the 4.x functionality Matrix the MapImageLayer is marked as
(not to be confused with the 4x MapImageLayer which is the 4x version of the 3x ArcGISDynamicMapServiceLayer)
Is there any update to this? I too am looking for a way to use 3.x mapImage in 4.x. Specifically, I would like to load an image using extents, like this. If 3.x mapImage is still not ported over to 4.x, is there another way to approach this in 4.x?