Both Olympus E-M10 IV and Pentax K-S1 have 20.0 MP resolution sensors but Olympus E-M10 IV's sensor is Four Thirds (17.4 x 13 mm ) and Pentax K-S1's sensor is APS-C (23.5 x 15.6 mm ).
Since Pentax K-S1'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 Olympus E-M10 IV.
One other difference between these two cameras that is worth mentioning is that Pentax K-S1'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 K-S1 sensor size comparison.
As seen above, Pentax K-S1 has a 1.6x 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.