Nikon S7000 has a
16.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Nikon S4000 has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Expeed C2 processor.
Below you can see the S7000 and S4000 sensor size comparison.
Nikon S7000 and Nikon S4000 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 Nikon S4000 has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Nikon S7000, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.