Files with the C4 extension are used to store compressed bitmap graphics. CCITT Group 4 encoding is used to create C4 files. The image structure is very similar to the TIFF or TG4 graphic format. It is a format consisting of properly indexed tiles. The C4 format comes in different variants and not all files with such an extension can be supported by popular graphics programs.

This format was used by the United States Department of Defense. Currently, this format is rarely used and is considered obsolete.