Nikon S4100 has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Expeed C2 processor. On the other hand, Canon SD780 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the S4100 and SD780 IS sensor size comparison.
Nikon S4100 and Canon SD780 IS 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.