HDF4® is a library and multi-object file format for storing and managing data between machines. HDF4 is a very different technology than HDF5. The HDF Group recommends that, other than for working with existing HDF4 data, new applications use HDF5 since HDF5 addresses important deficiencies of HDF4.
NASA maintains a network of Earth Observing Satellites that transmit huge volumes of new imagery data daily in HDF4. As a result, HDF4 is used extensively in the Earth Sciences. The HDF-EOS Website, http://www.hdfeos.org has a wealth of information on these uses.