Olympus E-M10 IV has a 20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic VIII processor. On the other hand, Leica M9 has a 18.0MP Full frame (36 x 24 mm ) sized CCD sensor .
One other difference between these two cameras that is worth mentioning is that Leica M9's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the E-M10 IV and M9 sensor size comparison.
As seen above, Leica M9 has a 3.8x Larger sensor area than Olympus E-M10 IV. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.