noun Definition of g4 in Technology (protocol, compression) (G4) The CCITT fax protocol which uses data compression and allows a variety of file types (e-mail, pictures, PostScript, etc.) to be transmitted over digital (ISDN) telephone lines. The Group 4 protocol was published by CCITT in 1993. Full details of the protocol are available from ITU-T. See also Group 3. 1
