Has anyone with a WD NAS formatted drives for another system?

If you don't care what's on the drives, just replace the partition table. I have no idea why WD would use unpartitioned space on the drive for storage, but weird behavior is par for the course for proprietary systems, so it certainly doesn't surprise me. On Linux, you can run fdisk -l as root to doublecheck everything; you should see output along the lines of

$ sudo fdisk -l
Disk /dev/sda: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: Samsung SSD 860 
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: <UUID>

Device       Start        End    Sectors   Size Type
/dev/sda1     2048     194559     192512    94M EFI System
/dev/sda2   194560    2148351    1953792   954M Linux filesystem
/dev/sda3  2148352 1953523711 1951375360 930.5G Linux filesystem


Disk <etc>

From what you're describing, it sounds like you should expect something along the lines of Disk /dev/whatever: 4000 GiB, 4<trillion> bytes, <many> sectors with partitions not adding up to the full size of the drive. If so, creating a new partition table with a single partition covering the entire drive will give you access to the full amount of storage (though will delete any data currently on the drive).

So I had the exact same problem when my WD PR2100 NAS died on me a few years ago. Unfortunately, the solution was to get another drive and move the data from my WD NAS to a new disk manually, then format the drives in the NAS before moving them to the new system and copying the data to the new NAS.

Do you have an extra drive that you can spare as a temporary data store to transfer the files between systems?

What kind of partition type are the drives formatted in? MBR is limited to 2TB max, which could explain what you're seeing. If the drives aren't using MBR, then it could be that your WD NAS used some proprietary partitioning setup for backup/redundancy, which may have needed a certain amount of unallocated space on each drive in order to function. And it certainly wouldn't be the first time I have seen a consumer grade NAS do something like that. :P

On the plus side, regardless of why your partitions are so small (unless the drives are damaged), using a partition manager you should be able to convert the drives from MBR to GPT (which supports larger drives), expand the volumes using any unallocated space, and/or merge the volumes if there are multiple per HDD. I personally use AOMEI Partition Assistant for Windows, since it's comprehensive, incredibly easy to use, GUI based (I hate using CLI partition managers), and even the free version should have everything you need. But you should be able to do all the above with pretty much any partition manager too, so just find one that appeals to you and look for instructions on how to use it. Only certain ones, like AOMEI PA, can convert partition types and merge volumes without losing data though, so YMMV depending on which you choose and what you need to do.

To expand on what others have said, there's some interesting info here on a workaround for using >2TB drives on systems that only support MBR, so it's possible that the WD enclosure was doing some sector size trickery that isn't being recognised by the software you're using.

If you can temporarily store the data elsewhere, creating a new GPT partition table should do the job.