Both Sony HX400V and Sony RX10 IV have 20.0 MP resolution sensors but Sony HX400V's sensor is 1/2.3" (6.17 x 4.55 mm ) and Sony RX10 IV's sensor is 1 (13.2 x 8.8 mm ).
Since Sony RX10 IV'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 HX400V.
Below you can see the HX400V and RX10 IV sensor size comparison.
As seen above, Sony RX10 IV has a 4.1x Larger sensor area than Sony HX400V. 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.