Inexpensive NAS for over 4TB backups - recommendations?

allowing to delete files will give you exact backup but will also introduce an error risk. If someone deletes a wrong file on the source, it will be deleted from the backup after the sync too. This means the only recoverable source would be the snapshots on the 104 if enabled. Those snapshots are not definitive and will be erased automatically after a certain time or when disk is full. Moreover if you rely only on one device, you may have a failure with that device too and you may not be able to use the snapshots at all.
The choice is whether you are willing to take the risk to lose a file from time to time or if you prefer to maintain a really big save which will get messy. The choice partly depends on who is operating the NAS, if everyone is responsible and will say immediately when they delete something they don't want, it's pretty sage to use delete option, if you are dealing with daydreamers then it may not be. You may also want to backup yearly used files since they are prone to be forgotten and may be out of time for snapshots.