- An Enigma machine is any of a family of related electro-mechanical rotor machines used for the encryption and decryption of secret messages. The first Enigma was invented by German engineer Arthur Scherbius at the end of World War I.
- GNU m4 is the GNU version of the m4 macro preprocessor. It is designed to avoid many kinds of limits found in traditional m4s: limits like maximum line lengths, maximum size of a macro, number of macros, etc. Removing such arbitrary limits is one of the stated goals of the GNU projects.
- The M4 carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16, all based on the original AR-15 designed by Eugene Stoner and made by ArmaLite. It is a shorter and lighter version of the M16A2 assault rifle, with 80% parts commonality.