Protocols and standards
Group 4
Group 4 is the class of digital fax machines designed for ISDN, which sent a page in a few seconds at 64 kbit/s, without a modem.
Standardised in the 1980s, it relied on an end-to-end digital link and MMR compression, with higher resolutions than Group 3. It never went beyond a few professional uses: it required ISDN access at both ends, and Group 3 kept getting faster in the meantime.
Why it matters for fax
Group 4 disappeared with ISDN. A fax today is always a Group 3 fax, even when it comes from software or a server: it is the only language spoken by all the fax machines still in service.