Data formats and access tools necessarily change as technology improves to address emerging requirements with new capabilities. This on-going process inevitably leaves behind significant data collections in legacy formats that are difficult to support and sustain. NASA ESDIS and The HDF Group currently face this problem with large and growing archives of data in HDF4, an older version of the HDF format. Indefinitely guaranteeing the ability to read these data with multi-platform libraries in many languages is very difficult. As an alternative, HDF and NASA worked together to create maps of the files that contain metadata and information about data types, locations, and sizes of data objects in the files. These maps are written in XML and have successfully been used to access and understand data in HDF4 files without the HDF libraries. While originally developed to support sustainable access to these data, these maps can also be used to provide access to HDF4 metadata, facilitate user understanding of files prior to download, and validate the files for compliance with particular conventions. These capabilities are now available as a service for HDF4 archives and users.