Prospecting results should be similar for 4 chunks in an area because the ore densities are created from a density map generated with the world, at a resolution of approximately 32 blocks. The ore densities will tend to have a more concentrated 'center' and then decrease when moving towards the circumference. So when locating a low density reading of any desired ore, try taking other prospecting samples some distance away in other directions to find an area with a higher reading. With enough sampling, a player can locate the center of the ore field, which often span a great many chunks.
