Hard bundle means that the hard drives are already inserted in the NAS. Soft bundle means that the hard drives are delivered separately to the NAS. I recommend a DS418, it is easier to expand and offers much better protection against hardware failures, since a RAID5 can be configured with 3 hard drives.

Equipped with cheaper 3TB drives, a RAID5 would even provide 6TB instead of 4TB and there would still be a hard drive slot for further expansion.

Synology DS418

WD Red Plus (3 TB, 3.5", CMR)