Hi
Cool that you’re up & running…
Your Disk may “appear” as full, but you’re forgetting the local-lvm…
Proxmox defaults:
-> Part of the disk (usually 100GB) are formatted as a “normal” OS disk volume. All the Rest is local-lvm. This space can be used for VMs / LXC (LinuX Containers). See in the GUI, on the right side under local-lvm. Most space is free! But is not mounted as a file system, this is a block storage device…
You can mount the 4 TB disk as needed (I’d do just one partition, XFS, easier for starters).
But to be honest, I advice using NAS storage (Faster, more flexible, and easier to handle if you’re not yet really into linux) !
See my WIP Proxmox Advice here:
https://wiki.nethserver.org/doku.php?id=userguide:nethserver_and_proxmox
Still work in progress, but you may gain a few insights… 
I’d suggest allocating 100 GB for starters for your NethServers, and enlarge as needed.
Andy