Both Sony RX10 IV and Olympus E-M1 II have 20.0 MP resolution sensors but Sony RX10 IV's sensor is 1″ (13.2 x 8.8 mm ) and Olympus E-M1 II's sensor is Four Thirds (17.4 x 13 mm ).
Since Olympus E-M1 II's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Sony RX10 IV.
One other difference between these two cameras that is worth mentioning is that Olympus E-M1 II'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 moiré occurring in certain scenes.
Below you can see the RX10 IV and E-M1 II sensor size comparison.
As seen above, Olympus E-M1 II has a 1.9x Larger sensor area than Sony RX10 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.