1994.06.21 20:37 "CCITT (ITU) Group 4 raw data", by Shimon Gonen

1994.06.21 21:38 "Re: CCITT (ITU) Group 4 raw data", by Bill Cattey

The conversion from Group 4 fax is also there.

The Group 4 fax decode (which I have been inside of) and presumably the group 4 fax encode (which I take on faith having seen group 3 encode) looks at slightly different (and simpler) header bits, and then calls the Group 3 fax decode (or encode).

It's all there in tif_fax3.c