Japanese Apple blog Macotakara reports that Hitachi and Sony have already begun shipping 4″ displays for a future iOS device, as well as panels for the iPad 3.
The report, pointed out by MacRumors, also suggests that Hitachi Displays and Sony Mobile Display Corporation will be merging with toshiba Mobile Display Co., Ltd. to form “Japan Display” in Spring of 2012.
While the report doesn’t indicate which iOS device the screens would be used for, the most obvious answer woud be the iPhone, as Apple has already reportedly been investigating 4″ displays for the next-gen iPhone.