The segmentation 4D nrrd file is considered Slicer-specific, to enable single-file export/import of whole labelmap based segmentations. So there are no tutorials, because we don’t intend this to be used directly, but through the Slicer storage infrastructure.
The extent does not start at 0 because segments do not store the whole reference image geometry, only the non-empty part of the segment labelmap (“effective extent”).
(The vtkImageData::Extent variable that you linked just now explains what an extent is, but it does not explain the effective extent used for segments)
I suggest that instead of trying to read this 4D nrrd, you export the segmentation as a labelmap within Slicer and save that in a usual 3D nrrd. Of course any overlap information will be lost (if you’re concerned about this you can export the segments in individual labelmaps too with some python scripting).
There are many topics describing how to do this programmatically, such as this or this. If you want to do this from the UI then here’s the easiest way
but you can find more options in the Segmentations module