In spherical mirrors, sign convention is crucial. Distances are measured from the pole. Object distance (u) is always negative for real objects placed in front of the mirror. Image distance (v) can be positive (for virtual images behind the mirror) or negative (for real images in front).

Given a real object 4 cm in front, its object distance is $u=-4\mathrm{cm}$. The image distance depends on the mirror type and focal length.

Therefore, the correct statement is: object distance is always negative.