CCITT Group 4 compression, also referred to as G4 or Modified Modified READ (MMR), is a lossless method of image compression used in Group 4 fax machines defined in the ITU-T T.6 fax standard.
G4 offers a small improvement over G3-2D by removing the end-of-line (EOL) codes.
G3 and G4 compression both treat an image as a series of horizontal black strips on a white page.
Both G3-2D and G4 add a two-dimensional feature to achieve greater compression by taking advantage of vertical symmetry.
A worst-case image would be an alternating pattern of single-pixel black and white dots offset by one pixel on even/odd lines.