Trouble with GUID HDD recovery in testdisk
Initially partitioned with Disk Utility (as GUID pt, 1 partition, HFSJ) I would expect the partition tables of your WD Red 4 TB disks to look like this:

     start         size  index  contents
         0            1         PMBR
         1            1         Pri GPT header
         2           32         Pri GPT table
        34            6         
        40       409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
    409640   7813365344      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
7813774984       262151         
7814037135           32         Sec GPT table
7814037167            1         Sec GPT header


with the EFI = 409640 blocks and the empty default Apple spacer of 262144 blocks. All aligned to the 4096 Byte physical block size the resulting HFSJ partition starts at block 409640 and has a size of 7813365344 blocks.

You may just test this by removing the current partition (below I assume the disk identifier of the external drive is disk2 - use the disk identifier you have found in your environment):

diskutil umountDisk /dev/disk2
sudo gpt remove -i 1 disk2
sudo gpt add -i 1 -b 40 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B disk2
sudo gpt add -i 2 -b 409640 -s 7813365344 -t 48465300-0000-11AA-AA11-00306543ECAC disk2


If asked don't initialize any volume.

Now verify the volume:

diskutil verifyVolume disk2s2




After further investigations it turns out that one of the two external disks contains an additional recovery partition. After adding a 3rd partition like this:

     start         size  index  contents
         0            1         PMBR
         1            1         Pri GPT header
         2           32         Pri GPT table
        34            6         
        40       409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
    409640   7812357952      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
7812767592      1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
7814037128            7         
7814037135           32         Sec GPT table
7814037167            1         Sec GPT header


and the help of Disk Warrior (to restore broken file systems) both main volumes of the external disks have been salvaged.