Nikon Z50 has a 21.0MP APS-C (23.5 x 15.7 mm ) sized BSI-CMOS sensor and features Expeed 6 processor. On the other hand, Nikon 1 J4 has a 18.0MP 1-inch (13.2 x 8.8 mm ) sized CMOS sensor and features Expeed 4 processor.
Another difference between these two cameras is that Nikon Z50's sensor lacks 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 Z50 and 1 J4 sensor size comparison.
As seen above, Nikon Z50 has a 3.2x Larger sensor area than Nikon 1 J4. 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.