Megapixel Count versus Image Resolution
When you upgrade from a 4MP sensor to an 8MP sensor, how much more resolution do you get? Double? Not quite.
The megapixel count comes from multiplying the length and breadth of an image to get its area. But resolution is measured linearly. While the surface area of the image doubles from 4MP to 8MP, the length and breadth of the image only increases by around 45% each.
To get an actual doubling of image resolution, you have to quadruple megapixel count. So to get double the length and breadth of a 4MP picture, you actually have to shoot with a 16MP sensor.