The flange to film distance of the M42 screw mount, for example, is
45.46mm.

A four-thirds sensor is about half the size of a 24 by 36mm full-frame
sensor.

Half of 45.46 is 22.73.

The regular Four Thirds system has a flange to film distance of 38.67mm,
considerably larger than that... but the flange to film distance of
Micro Four Thirds is 19.25mm. Too small, apparently, by 3.48mm.

So neither of those lens mount systems is optimal for making the most
compact possible DSLR built around a four thirds sensor.

John Savard
http://www.quadibloc.com/index.html