Some applications likely will. If they use new Hildon APIs (such as PannableScrollArea or AppMenu), they'll work on an N8x0 in Mer. If they stick to the current Hildon API, they'll work on both an N8x0 and Rover. However, any apps using Clutter, OpenGL ES and so on will not work on a Maemo 4 device (lack of horsepower, no driver for accelerated 3D and APIs not present). Mer can fix one of those :-/