Pentax X90 has a 12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Prime processor. On the other hand, Sony A7R IV has a 61.0MP Full frame (35.8 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz X processor.
Sony A7R IV's sensor provides 49MP more than Pentax X90'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.
One other difference between these two cameras that is worth mentioning is that Sony A7R IV'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 X90 and A7R IV sensor size comparison.
![Pentax X90 vs Sony A7R IV Sensor Size Comparison]()
Sensor Size and Resolution Comparison image of Pentax X90 and Sony A7R IV Cameras
As seen above, Sony A7R IV has a 30.7x Larger sensor area than Pentax X90. 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.