Digital zoom with S4

Most camera reviewers seem to think that digital zoom is of little use and suggest that you might just as well crop the image on your PC.
But is this true?
If I'm shooting at maximum resolution it must be so but if I'm shooting at less than maximum then there should be some scope for the camera to use the 'unused' resolution to zoom digitally without loss of quality.
Does anyone know if digital cameras in general, and the S4 in particular, actually do this?

My understanding of the digital zoom on these cameras is that the central part of the image is recorded (same as cropping the final image) and there is no trade off with unused 'capacity' because you are not shooting at maximum resolution. Anyway the answer in the situation you describe would surely be to shoot at maximum res and crop the final image.
With my S4 I usually shoot at maximum res and crop as need be - results are excellent.

Also, the digital zoom works in movie mode whereas optical zoom does not.

matt