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, Sigma fp has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor .
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 moire occurring in certain scenes.
Below you can see the X-T4 and fp sensor size comparison.
As seen above, Sigma fp has a 2.3x Larger sensor area than Fujifilm X-T4. 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.