Both Olympus E-M10 IV and Panasonic GX9 have Four Thirds sized 20.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
One other difference between these two cameras that is worth mentioning is that Panasonic GX9'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 GX9 sensor size comparison.
Olympus E-M10 IV and Panasonic GX9 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.