Sony A6400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Sigma Quattro has a 29.0MP APS-C (23.5 x 15.6 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor. Sigma Quattro's sensor provides 5MP more than Sony A6400'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.
Below you can see the A6400 and Quattro sensor size comparison.
Sony A6400 and Sigma Quattro have 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 Sigma Quattro has 22% larger pixel area (18.69µm2 vs 15.28µm2) compared to Sony A6400, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.