Works with a plain adapter
Four Thirds sits 10.87mm further from the sensor than Leica M. You need a 10.87mm adapter, but it can be a plain one — a hollow metal spacer with no glass in it. The lens still reaches infinity focus and nothing is added to the optical path.
Coverage & framing. Four Thirds lenses are built for a Micro Four Thirds imaging circle, which is smaller than Full frame. Expect heavy vignetting or a forced crop on a Leica M body.