HDF4 (originally known as HDF) is a file format for storing scientific data and
a software library that provides high-level APIs and a low-level data interface.

HDF technologies at present include two data management formats (HDF4 and HDF5)
and libraries, a modular data browser/editor, associated tools and utilities,
and a conversion library. Both HDF4 and HDF5 were designed to be a general
scientific format, adaptable to virtually any scientific or engineering
application, and also have been used successfully in non-technical areas. The
Open Source format is a key technological foundation for HDF core technologies.
It allows users to collaborate with The HDF Group regarding functionality
requirements and permits users' experience and knowledge to be incorporated into
the HDF product when appropriate. It also permits users and organizations
without adequate technology resources to use a sophisticated and robust data
management tool for no charge.

WWW: http://www.hdfgroup.org/products/hdf4/