Both Olympus TG-860 and Olympus TG-4 have 16.0 MP resolution sensors but Olympus TG-860's sensor is 1/2.3" (6.17 x 4.55 mm ) and Olympus TG-4's sensor is 1/2.3 (6.17 x 4.55 mm ).
Since Olympus TG-4's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Olympus TG-860.
Below you can see the TG-860 and TG-4 sensor size comparison.
Olympus TG-860 and Olympus TG-4 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.