Individual raster datasets
A mosaic dataset creates a seamless view of two or more adjacent or non-adjacent images. In a mosaic dataset, all rasters have the same cell size, format, and coordinate system.
A raster catalog is a table that groups two or more raster datasets. In a catalog, rasters may have different cell sizes and different formats, but they must have the same coordinate system.
For more details about the ECW raster format, refer to http://www.hexagongeospatial.com/products/data-management-compression/ecw.
All those four rasters are stored...