Sony RX100 IV has a 20.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Sony RX1 has a 24.0MP Full frame (35.8 x 23.8 mm ) sized CMOS sensor .
Below you can see the RX100 IV and RX1 sensor size comparison.
As seen above, Sony RX1 has a 7.3x Larger sensor area than Sony RX100 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.