Test image: 3D+t Heart

If you perform a dynamic segmentation using the Watershed tool, it creates a new segmentation no matter if you created a dynamic segmentation before.
The dynamic segmentation stays 4D (which is correct) but the new '_Watershed' segmentation is 3D (bug).
The result is stored in that new segmentation not the dynamic one.

To not blow up the data manager with this, the _Watershed should not be created but instead the dynamic segmentation should be used. In the dynamic case, a 4D segmentation should be the result of the Watershed tool.