Never heard of the Quarter-millimeter before. Strange mix of divisions by tens suddenly switching to divisions of twos
Digging deeper, the kyu -- or Q for quarter millimeter -- is apparently a foundational distance measurement in Japanese typesetting, which is metric and operates on a millimeter grid.
> the German and Japanese point is 0.250 mm
It's probably the sanest adaptation of the point to the metric system. A traditional point is close to a third of a millimetre, but that's too weird.
Since the Q is close to 3/4 of a traditional point, it's also quite easy to convert from traditional multiple-of-three point sizes: 9 pt -> 12 Q, 12 pt -> 16 Q, etc.
Although it's even easier just to call those 3 mm and 4 mm!
;)