4TB HDD accidentally formatted 1.8TB and now can't repartition as 4TB!

I recently purchased a SEAGATE 4TB spinning HDD (ST4000DM004) and put it inside an old NewerTech Voyager Q drive dock (many years old and with FireWire, but yet which still works perfectly with my 1TB drives), and I attached that dock to a late 2015 5k iMac via FireWire800-to-TB cable. I ran High Sierra's Disk Utility and the 4TB drive only showed up as 1.8TB. That worried me, but I went ahead and formatted it to see what would happen, and I only got 1.8TB, confirmed. I then realized the chips in my old drive dock would be the reason, so I bought a new drive dock that supports drives up to 8TB. In Disk Utility, in the Partition section, I see 1.8TB in blue (pie chart) and 2.2TB free (4TB total, proving my new drive dock is OK). But Disk Utility throws an error when I try to partition it as 4TB!

So I opened the terminal and tried this:

diskutil partitionDisk /dev/disk2s2 GPT JHFS+ New 4000g

But that gives me this error:

/dev/disk2s2 does not appear to be a whole disk

I then tried this Terminal command, but it gives me the same error too!

diskutil partitionDisk disk2s2 1 GPT HFS+ newdisk R

When I do "diskutil list" I see this:

/dev/disk2 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *4.0 TB disk2

1: EFI EFI 209.7 MB disk2s1

2: Apple_HFS 4TB_HDD 1.8 TB disk2s2

Can anyone please help me reformat/repartition my HDD as a single 4TB volume (under High Sierra)? Thank you.