Fujifilm X-T4 has a
26.0MP APS-C (23.5 x 15.6 mm ) sized X-Trans CMOS 4 sensor . On the other hand, Sony A580 has a
16.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz processor.
Fujifilm X-T4's sensor provides 10MP more than Sony A580'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 Fujifilm X-T4'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 moiré occurring in certain scenes.
Below you can see the X-T4 and A580 sensor size comparison.
Fujifilm X-T4 and Sony A580 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Sony A580 has 61% larger pixel area (22.87µm2 vs 14.12µm2) compared to Fujifilm X-T4, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.