The My Cloud PR4100 is a completely DIFFERENT model/unit than the single bay My Cloud units.
WD indicates at this link: http://www.wdc.com/en/products/nas/pro/ that the PR4100 does support third party apps.
You should probably see the WD Pro Series subforum and search/ask in there about NZBGet for the PR4100 unit.
https://community.wd.com/c/network-attached-storage/wd-pro-series
NZBGet is mentioned in the following two WD Pro Series threads:
https://community.wd.com/t/docker-and-apps/170177
https://community.wd.com/t/sleep-lcd-and-apps-on-pr4100/170085
