When configured for the low resolution CGA Mode 4, the display memory is organized in a similar fashion to that of Mode 6 except that each byte in the scan line is organized as four 2-bit pairs. Each pair corresponds to one of the 320 pixels in the scan line. The two bits in each pair provide sufficient information to specify 1 of 4 "colors" (grey shades) for the corresponding 2-bit pixel. The most significant bit-pair corresponds to the left-most pixel (dot-pair) on the screen. The most significant bit in each pair is most significant in determining the shade, with larger values corresponding to darker pixels. Note that it is possible to reverse the order of the shades.
Even Scan Lines
Odd Scan Lines
