I expect every one knows this but me. I have been examining the format of mode 4 sprites. Why you say. Because I don't have a clear written description of it, even although the format was set 20+ years ago. Anyway to my surprise the line width of the sprite regardless of the visible pixel width seems to be a multiple of 4 bytes every time. Even a 7 pixel wide sprite in 4 colours which uses 2 bits to map each of the 4 colours available per pixels has a minimum line width in bytes of 4 not 2.
Why I wonder. Its a bit costly in memory in a system that was memory lite - 128k in a black box QL with about 96k available once the system had been loaded. In fact memory was so tight that some applications used the screen memory as a work area when processing data.
Anyway memory is no longer an issue. I wonder if this apparent profligate use of memory has allowed the Window Manager to be adapted to use 24 bit sprites although only 16 bit colours can be displayed.