Juan,
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
Coming soon
(not to be confused with the 4x MapImageLayer which is the 4x version of the 3x ArcGISDynamicMapServiceLayer)