Sony A7R IV has a 61.0MP Full frame (35.8 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Fujifilm GFX 50R has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X Processor Pro processor.
Sony A7R IV's sensor provides 10MP more than Fujifilm GFX 50R's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Another difference between these two cameras is that Sony A7R IV'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 A7R IV and GFX 50R sensor size comparison.
As seen above, Fujifilm GFX 50R has a 1.7x Larger sensor area than Sony A7R 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.