You cannot use a ZFS pool directly as a storage device, you’ll always need a dataset or volume. That being said, every pool has a root dataset with the same name as the pool. It is not a good practice to store data in the root pool in m opinion, you should create a sensible dataset hierarchy instead. If you really need to, you can even reorganize this hierarchy at a later point.
Leave the filesystem and formatting up to the guest system.
Since you mentioned you have 4x 2 TB drives, I assume you are using Raid-z1. This will give you a total pool capacity of 6 TB (Terabytes, decimal prefix) = 5.5 TiB (Tebibyte, binary prefix). When using zpool list or zfs list, ZFS will use the binary prefix system, so 5.5T means 5.5 TiB, therefore the number shown by ZFS is correct in your case.
As to where the discrepancy comes from, I have no idea. I don’t use XCP-ng so I don’t know where the “4.8TB” would be shown and what it refers to. Could it simply be the remaining free storage after you installed some VMs?